Irving Azoff, founder of the licensing management firm Global Music Rights (GMR), alleges the web giant does not have the performance rights to about 20,000 works by the artists he represents.
Google, which owns YouTube, doesn't appear to be backing down.
In a letter posted online, Google lawyer David Kramer called GMR's legal position "misguided," and argued the firm hasn't even properly identified the works that are at issue.
A lawyer representing GMR told the Hollywood Reporter that if Google doesn't blink, "there will be a billion-dollar copyright infringement lawsuit filed."
Streaming sparks legal war
Azoff has been turning up the pressure on YouTube since November, when the web giant launched its Music Key subscription service to rival other digital music outlets like Spotify and Pandora.
"The way fans listen to music is evolving daily," Azoff told the Hollywood Reporter last month.
"The trampling of writers' rights in the digital marketplace without any regard to their contribution to the creative process will no longer be tolerated."
Google hasn't publicly responded to GMR's claims which haven't been proven in court.
Royalties have become more important as the music industry moves away from traditional album sales, and even digital downloads, to an online streaming based model.
The world's three largest music labels — Universal, Sony and Warner — all have reached licensing deals with YouTube as part of its new subscription services. Hundreds of independent labels, including some that had been holding out for better terms, also are on reported to be on board.
YouTube is expected to sell an estimated $7.2 billion in advertising this year, though Google has never disclosed how much revenue flows through YouTube.