I have the same issue, I can reimport the sound and it starts to work again. I believe my issue is caused by having multiple music managers (they are different names). I have one that was placed into the unity scene view (the first scene of the game) which then persists amongst every other scene in the game. I make it disable the audio source component when it enters levels with a different sound manager and re-enable it again on scenes without said managers. If I go through scenes during testing in unity and back to the first scene a few times this error shows up and wont play my menu music any more. I think reloading the project fixes the issue, until it arises again.