Jens, I must say I have no clue what's happening... It is somehow related to system resources Vox uses, which other audio apps don't use. Which music apps You say are working fine do emerge from Mac Appstore and not from Apple?
I made this Sandbox-less version especially for You, I am not sure it will work in Your case, however, please try it:
https://rink.hockeyapp.net/apps/af118bfd0a08458e912d7f39d81619a4/ap...