did you install a Mod ? If yes, then your issues likely come from that.
Else, do you have the AppStore or ManaGames.com version ?
If the AppStore, read here, maybe it could help => topic29-15863.php
Else, I have really no idea what could be wrong, especially if sometimes it works, and sometimes it doesn't... It could be an issue with the audio drivers of your Mac Mini or something like that.