I've been adjusting my amp volume way up whenever I play MK3 to counter its horridly low volume. Always thought it was an emulation thing. Then I ran the test mode, and lo and behold it had a volume adjustment right there! It was, of course, only half way up.
Then I noticed you can map volume buttons too. Never noticed them before in the tab menu.
Just thought I'd help out anyone else who's still playing with MK3 on half volume and hasn't tried the test mode menu yet.

I bet something similar solves the low volume problem in a couple other quiet games too.