It's only a guess, but I would assume Kawaks uses some sort of automagic normalisation like a lot of emulators do (I know AdvanceMAME does the same). MAME by definition is against that sort of thing, but you can still fiddle with the options yourself.
During a game, whack the tilde "~" button. Each time you push it you'll be presented with a new menu. There are things like sound channel volume options (including qsound stuff for those games), gamma controls, etc. Try fiddling with those and see if you can make it sound a little better.
Remember that for better or worse, MAME is not intended to modify a game in any way. If the original board had poor sound output, then so will MAME. Any post-processing (video or audio) is not the job of the MAME devs. Hence why you don't see all the fancy scale2x modes and whatnot in vanilla MAME.