It's probably your processor. It's not bad for older games, but newer games, when the processor is maxed out, will repeat the sounds.
Also, some of the games you mentioned (outrun, and other sega games) are in the middle of a rewrite.
last, mame is great. But for speed, it's not the best. If you find another emulator that runs a game, it will probably run it faster. So look around at some of the others
system16
raine
m72
retrocade (some games its great at, others.... well...)
hive (will play old games on a 486)
finalburn (for the older games)
finalburn alpha (for cps2)
there is a great guantlet emulator I used when I had a 450... but didn't have a speed limiter so I went to mame. Still it looks better then mame does
Rygar has a great single emulator REM! that still plays better then mame... but again, not speed limiter
I'm sure I'm missing some. After moving from a 450 to a 1ghz, I stopped caring about most other emulators since mame could run 98% with full fps.