There was a time when MP3 files were considered by the record labels as something akin to spawn of the devil. So evil, in fact, that in 1999 the RIAA tried pressuring a search engine portal to remove an MP3 search feature.

Now there’s digital rights management technology, with the two most notable protection envelopes being FairPlay, which is used to protect iTunes music, and Windows Media, which is used on just about everything else.

The trouble is that the two main DRM technologies are incompatible with each other, which is why protected WMA files will not play on iPods and protected AAC files will play on nothing but iPods. Added to that confusion is the new flavor of Windows Media that Microsoft recently introduced, which only plays on the company’s new Zune personal player.

Plain old MP3 files play on all players, including Zunes, iPods and whatever you can find on the shelf of your local Circuit City. Of course, MP3 files don’t carry any kind of DRM technology, which is why the labels have avoided using them.

But it’s that very same reason that’s now causing the labels to take another look at MP3. Blue Note Records is selling Norah Jones’ latest single, “Thinking About You,” as a music download on Yahoo, and Capitol Records is selling two MP3s by Christian band Relient K. Because Blue Note and Capitol are part of the EMI Music Group empire, the move represents an about-face by a major label.

Why now? The most obvious reason is the labels are concerned that Apple is getting too powerful, with iPods and iTunes forming the current center of the legal download universe.

Called “an experiment” by Blue Note General Manager Zach Hochkeppel, the Norah Jones MP3 isn’t expected to cut into sales of her upcoming album, also called Thinking About You, which is due out next month. But that’s mainly due to Jones’ mostly adult fanbase, which is less likely to download forbidden musical fruit, according to The Wall Street Journal.

“Nobody gets hurt – we think,” Hochkeppel said.

MP3s traded via file-sharing networks have been the scourge of the recording industry ever since the original Napster went online in 1999. However, there is a growing feeling among industry execs that the only companies benefiting from DRM technologies are companies like Apple, whose proprietary copy-protection locks iTunes customers into using iPods.

Meanwhile, competitors of Apple’s iTunes / iPod combo have had to deal with players created by several different companies. But if someone started buying music downloads through iTunes, that person is not likely to dump all the music accrued over the years by switching to a non-iPod player.

Then there’s eMusic, the No. 2 online music store in the biz, which sells nothing but MP3s. And thrives. That’s something the majors might want to consider the next time they negotiate prices with Apple.