I just upgraded from an old MacBook running 10.6 to a slightly less old MacBook Pro unibody (mid-2009) running 10.8. I find that, very often, when an app triggers the system alert sound -- for me this happens most often in iTerm2 and MacVim -- it can take something around half a second to a full second from the time the sound "should" occur (e.g. when I hit an unrecognized key or try to move the cursor past the end of a file) to the time it actually does.
Is there a way I can make the sounds happen instantly? I'm not sure if this is a hardware or software issue, but it doesn't seem to be limited to just one app.
EDIT: I just booted 10.6 on this machine, and the same delay is present there; so I know it's not a Mountain Lion issue.
EDIT 2: I now notice that when I have headphones on, I can hear a slight hiss through them if the system has played a sound in the last one or two seconds. After that, they go totally silent. If I try to trigger a sound event while this hissing is happening, there is no delay. So I think there must be something in a driver somewhere that "closes" the sound device after a short delay, and it takes a little time for it to start up again.
I also notice that if I've triggered several sound events during this silent period, I generally only hear one of them come out afterwards.