Some Hollywood actors think they’re so la-di-da. Or, more specifically in this case, do-re-mi.

A lot of ink has been spilled in recent months over the fact that unmusically inclined "Walk the Line" co-stars Reese Witherspoon and Joaquin Phoenix did their own vocals in playing country legends June Carter and Johnny Cash.

Now Demi Moore is reportedly learning to sing for her role in the Robert Kennedy assassination movie "Bobby."

But can people with no musical background really be taught to hold a tune and pull it off successfully … say, successfully enough to get murmurs of approval from the crowd at Wednesday night’s Grammy Awards?

Voice coaches believe singing can indeed be learned, even by the harmony challenged.

“I firmly believe that a person can be taught to sing for a role,” said Dan Manjovi, a vocal coach and performer who teaches singing at HB Studio, an acting school in New York.

But though Witherspoon and Phoenix received widespread praise for their performances — musical and dramatic — and others like the previously unmusical Renée Zellweger also impressed people with her vocals as Roxie Hart in "Chicago," there are those who believe the strategy usually hits a flat note.

“It rarely works,” said Seton Hall University film studies professor Christopher Sharrett. “The only time it genuinely works is if the actor has some musical ability and orientation.”

Witherspoon and Phoenix trained intensively for about four months, learning not only to sing but to play the autoharp in her case and guitar in his.

Zellweger spent 10 months of grueling song and dance training to ready herself for her role. And Moore has been learning two songs, including “Louie Louie,” in a Los Angeles recording studio, her representative recently told The New York Post.

“It was really, really hard,” Witherspoon said of doing her own vocals in an interview with Vogue. “I was so darn nervous I had to have a bucket nearby in case I was going to lose my lunch before I got up on the stage. And I tried to get out of it like six times. I said, ‘Jim [Mangold, the director], can’t you just get LeAnn Rimes to sing for me?'”

Manjovi, whose CD “Dan Manjovi” came out in the fall, said ideally, actors with no prior voice and music experience would need at least six months of hard work and coaching in order to be able to sing their way skillfully through a movie musical.

Training rookie vocalists is basically a matter of teaching them to overcome their fears about singing in public, helping them find pitches and smoothing out the gap between what’s called the upper part of the voice (which originates in the head and throat and is also called falsetto) and the lower part of the voice (which originates in the chest and is used for one’s natural speaking voice), according to Manjovi.

“[The upper part of the voice] is controlled by a completely different set of muscles,” he said. “What has to happen is the throat has to relax in order to get those muscles activated.”

And so while many students come in claiming they’re tone deaf, that usually isn’t what’s making them warble off-key.

“Tone deafness is a very rare phenomenon that involves people who actually have hearing problems,” said Manjovi. “What a lot of people mistake for tone deafness is a difficulty with finding pitches. I’ve taught them how to find those notes.”

But for some reason, it's the sheer fright surrounding singing that can hinder actors the most in fine-tuning a vocally challenging performance.

Some students of Juilliard voice coach Deborah Lapidus have been so terrified, she said, that they've had complete meltdowns when they've gotten up to do a song.

"People sob, they break down, they throw things," said Lapidus. "I had someone literally lose the ability to stand up. And I once had someone in class who started singing and then started screaming, 'I'm blind! I'm blind' [and had] hysterical blindness. We had to call health services."

Even in film — where imperfections can be cut, glossed over or helped along with recording technology — fear can be a factor. And part of that fear stems from criticism many people, even talented actors, have gotten since they were young about their singing voices.

Witherspoon, for her part, has described the experience of recording an album and filming the live concert sequences before hundreds of extras as "horrifying" — and has recalled a vivid childhood memory about singing for a role and being told to avoid ever trying it again.

"People have a lot of singing baggage. It's like an exorcism to teach someone to sing. You're exorcising the ghosts of all those negative messages," said Lapidus, who has helped actors including Sigourney Weaver, Jane Adams and Andre Braugher prepare for singing roles in "Snow White," "Happiness" and "Duets," respectively.

Weaver's part ended up being dubbed, which she was prepared for, according to Lapidus — who believes that even actors who will be lip-synching should take voice lessons to learn how singers use their bodies.

Mangold's approach to the musical biopic "Walk the Line" was to direct his actors to avoid making the project an exercise in mimicry — especially Phoenix, who was playing a country legend with a very recognizable voice.

On the other hand, "Ray" director Taylor Hackford approached the previous year's biopic about another iconic musician completely differently. Though leading man Jamie Foxx — who took home the Oscar for Best Actor — is musically trained, has singing talent and played his own piano for his role as Ray Charles, he lip-synched Charles' vocals throughout the film.

"There's never a perfect solution," said Chris Willman, a senior writer at Entertainment Weekly who was on the set of "Walk the Line" and interviewed the lead actors for his November cover story about the movie.

"It's almost like a can't-win situation. For the first 10 minutes in any movie like that, the audience is going to be distracted, whether they're thinking it doesn't sound quite right or they're distracted by the real singer's voice."

There have been famously brilliant and equally famously disastrous examples of both ways of approaching a role that requires singing.

Aside from "Walk the Line," other widely agreed-upon, fairly recent success stories are Gary Busey's performance as Buddy Holly in "The Buddy Holly Story" (1978), in which Busey sang the songs himself; Sissy Spacek as Loretta Lynn in "Coal Miner's Daughter" (1980), in which the previously musically trained Spacek did the vocals and was even nominated for a Grammy for the title song; Foxx as Charles in "Ray" (2004), in which the songs were dubbed but Foxx practically became Charles in his nearly-seamless handling of the role; and Kevin Spacey, who sang the songs as Bobby Darin in "Beyond the Sea" (2004).

Among the relatively recent performances that hit a sour — in some cases, very sour — note: Dennis Quaid, who lip-synched (many say jarringly) as Jerry Lee Lewis in "Great Balls of Fire!" (1989); Jill Clayburgh as opera singer Caterina Silveri in "La Luna" (1979); and Katharine Hepburn as Coco Chanel in the Broadway musical "Coco" (1969), for which she won a Tony and received mostly positive reviews for her acting even though her singing was lambasted by some with a trained ear.

Then there are movie stars who have tra-la-la-ed to mixed reviews: the previously untrained Zellweger in "Chicago" (2002), who got a lot of praise but some panning (her co-stars all had prior and fairly extensive musical experience; their vocals weren't as heavily scrutinized); and Ewan McGregor and Nicole Kidman in "Moulin Rouge!" (2001), who were taught to sing by the film's musical director but reactions were divided as to how well they pulled it off.

Even Witherspoon and Phoenix, whom many would place squarely in the "success" category, have their detractors as far as whether their voices did Cash and Carter justice.

Singing for a role is such a tenuous undertaking that some notables made a career of acting as a "voice double" for leading ladies.

One of the most famous: Marni Nixon, the uncredited voice double for Deborah Kerr in "The King and I" (1956) and "An Affair to Remember" (1957); for Natalie Wood in "West Side Story" (1961); and for Audrey Hepburn in "My Fair Lady" (1964). She even hit the high notes as an uncredited double for Marilyn Monroe in "Gentlemen Prefer Blondes" (1953).

Perhaps a new show called "Duets," created by "American Idol" judge Simon Cowell and rumored to be premiering this summer on FOX, will prevent Hollywood from ever returning to those voice-double days. Following the success of "Dancing With the Stars" and "Skating With Celebrities," the latest spin will see actors pairing up with professional vocalists and competing.

But with all the hurdles that can block an entertainer's inner singer from coming out, the journey is always bound to be accompanied by as many low Ds as high Cs.

"People tend to dance much more easily than they can sing or play the piano," said Sharrett. "It's not hard to teach a person to do a good jitterbug. But it's hard to get them to sound like Sinatra."