If yes, then you can change themes by pressing T (case sensitive). (looks like this key is not even documented, found it by accident)
You can even create custom themes. All themes are stored here /usr/share/moc/themes (manual said, that they should be in ~/.moc/themes , but that's not true)
The problem is that I'm unable to find a way to make it persist. Every time I restart moc it returns to default theme...
There's a way to change config file but I can't even find it.
Sound volume is okay on my machines.
Maybe you change software mixer in moc?
Try pressing x a few times to change the current mixer or you can try pressing w to disable software one.