Making it as a synonym will not end the problem as users will just add it without reading what it is for.
I think the only way is to clean the questions up and then burninate the tag
If we do not want to keep checking this tag is empty we will have to get the tag blacklisted.
I think volume is a difficult case as it will be the "obvious" tag to use when the question is written and they won't think of the other possible use.
It depends if we want to keep looking to check or we want to stop any abuse up front.If we are happing to keep checking I would make volume a synonym of audio as I suspect people looking for the filesystem use of volume will take a bit more notice of issues.