After years of selling online music digitally wrapped with copy and playback restrictions designed to hinder piracy, major music labels are beginning to make some songs available in the unrestricted MP3 file format.

The releases are part of an experiment to gauge demand for tracks that can be played on any digital music player capable of playing MP3s, one of the oldest music compression formats.

Normally, copy-protected tracks are only playable on certain devices. By selling MP3s, recording companies can ensure they can be played on Apple Computer Inc.'s (AAPL) market-leading iPod players without going through Apple's iTunes Music Store.

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The latest such offering comes from singer Norah Jones and rock band Relient K, both signed to labels operated by Britain's EMI Music.

Jones' "Thinking About You" and Relient K's "Must Have Done Something Right" — both from their forthcoming albums — went on sale for 99 cents each as MP3 downloads Tuesday via Yahoo Inc.'s (YHOO) online music service.

Relient K also is selling the song on the band's Web site with a bonus MP3 titled "Fallen Man," while EMI has released an MP3 from British pop singer Lily Allen for sale only in Great Britain.

Many artists, mostly with independent labels, have been selling music in the MP3 format for years. Dimensional Associates Inc.'s eMusic has sold nearly 100 million MP3 downloads to date.

Still, major labels have been reluctant to follow suit.

That is beginning to change, if slowly.

This summer, Sony BMG Music Entertainment released an MP3 by pop diva Jessica Simpson on Yahoo Music, while the Walt Disney Co.'s (DIS) Hollywood Records released an entire album by pop singer Jesse McCartney in the MP3 format.

"They're still looking at it as an experiment but the labels have really come a long way in terms of wanting to see how this works for them," said Carrie Davis, a Yahoo spokeswoman, refusing to disclose sales totals for the Simpson and McCartney tracks.