This has been reported in other threads. Fanburst is almost certainly the problem. They have become increasingly unreliable over the last year. This is out of Roll20's control. There is currently a feature that Pro users are testing on the dev server, that will allow you to upload your own music for playing. For now, the only thing you can do is hope that Fanburst fixes its problems, and rely more on Incompetech, Tabletop Audio or Battlebards.