The Mt. Lebanon, Pennsylvania, company has already sent formal warnings to some music sites, including MP3.com, one of the main hubs for downloading music files.

"We're highly confident in the validity of our intellectual property," said Scott Sander, chief executive at Sightsound.com. "We have two US patents that control the sale of downloadable music. We're not trying to slow [the Internet music industry] down. We're trying to speed it up."

But some companies said they'll challenge Sightsound.com's patents and put up a serious fight before paying licensing fees.

The brewing fight once again raises the question of how competent the US Patent and Trademark Office is in handing out exclusive rights to basic technologies underlying the Internet. If Sightsound.com's patents are upheld in court, it could exact a toll from the entire online music industry.

That prospect hasn't been lost on the traditional music industry. The Recording Industry Association of America, or RIAA, no friend to the online music movement, said Sightsound.com very likely will have to fight to get its money.

"At this point, the validity of these patents is almost certain to be challenged," said an RIAA spokeswoman. She declined to say whether the association would file suit.

Sightsound.com creates Web sites for other companies that want to sell music online. It claims its patents cover the idea of selling audio and video files through downloads. On Wednesday, Sightsound.com sent cease and desist letters to four Internet music companies including MP3.com, Platinum Entertainment (PTET), GoodNoise (GDNO), and Amplified.com.

Christopher Reese, vice president and general counsel for Sightsound.com, asked the sites to either pay a 1 percent royalty on all revenue from online music sales, or to "immediately cease and desist