If you are talking about music licensing deals for a specific game, usually they expire after a set amount of time, similar to certain assets like cars. Now if you are talking about the music itself, you have think about the composition rights and performance rights so to speak. The first is tied to the composer and the sheet music while the second could be tied to the people who performed in the actual recording . A particular performance could become abandonware if the master is lost, no reissues are made or the record company is acquired and its records get tangled in legal issues that prevent new reissues. For the second part, a musician can reconstruct the original composition sheet with enough time, but it would hard to replicate the original performance.