Google is cracking down on a popular site that converts the music from YouTube videos into MP3s.

YouTube

YouTube-MP3.org has received a letter from Google notifying site operators that converting videos this way violates YouTube's terms of service, according to the blog TorrentFreak, which said it has seen the letter.

YouTube, acquired by Google in 2006, has also apparently blocked YouTube-MP3.org's servers from accessing the video-sharing site.

"We have always taken violations of our Terms of Service seriously," said a YouTube spokesman, "and will continue to enforce these Terms of Service against sites that violate them."

The top four record companies have pressured YouTube for many months to do more to thwart the scores of MP3 conversion services. In December, the Recording Industry Association of America, the trade group representing the top record labels, noted the problem with these sites in a "report card" it issued on how well Google protects copyrighted material online.

Also, the RIAA wrote: "YouTube hosts videos explaining how to 'game' the Content ID system, and how to rip the audio content to create an MP3 file from a music video."

YouTube has spent big money to license music videos as well as enable users to incorporate music into their clips free of charge. Google, however, has also been one of the biggest supporters of free culture and a proponent of protecting the Internet from censorship.

It's a tough tightrope walk for Google to manage the company's interests in premium media while also defending an open Internet.

Whether Google's letter to YouTube-MP3.org is an isolated event or reflective of a larger campaign to boot conversion sites off YouTube remains to be seen. The way these conversion sites work is a user plugs in a YouTube URL into a tab field and the service creates an MP3 file of the music.

TorrentFreak says that the letter was written by Harris Cohen, a YouTube associate product counsel. He reportedly wrote that if the site continues to operate as is, it may result in "legal consequences."

Update 9:38 a.m. PT: To include comments from YouTube.