The service, which launched in the US, the UK, France and Germany yesterday, claims to have more than 25m tracks in its catalogue, making it four times the size of Apple's iTunes website.
However, Universal and EMI have said that licensing deals are not yet in place and they are still in negotiations with Qtrax, while Sony BMG has not confirmed anything.
said it will pay the record companies licensing fees, created by selling advertising space. McDonald's, Burger King, Samsung, Ford and Microsoft have already been signed up.
The company claims that the ads will not be intrusive and will not appear each time a song is played.
Warner Music is said to be negotiating with Qtrax after an earlier contract expired.
Qtrax allows users to tap into file-sharing networks to search for music, but downloads come with copy-protection technology to prevent users from burning songs onto CD.
Qtrax downloads can be stored indefinitely on PCs and transferred onto portable music players.
The service also promises that its tracks will be playable on Apple iPods and Macintosh computers as early as March.
Qtrax differs from other models that have attempted to do the same thing, such as Spiral Frog, because it has arranged licensing deals with all the leading recording labels as well as a number of independents.
James Bates, a director in Deloitte's media and entertainment division said: "Advertising will appear every time a song is played and that may prove too intrusive for many consumers.
"There is also a growing antipathy to online advertising amongst internet users generally, particularly targeted advertising.
"In addition, attracting sufficient advertisers that are interested in this type of advertising may also prove difficult."