This was actually noticed by a friend of mine, but if you load up the original Space Invaders (which is marked as working) which uses the mw8080bw.cpp source, the shooting sound seems to be incorrect.
Instead of making a "pew" noise, it sort of goes "peeeeee"
I know I sound like I'm rambling, so here's a video my friend did showcasing the problem and that it used to work on a very old version of MAME. He used 0.175 in the video to show the error, however I can confirm it is still a problem in 0.196
Hope you guys can fix it