By no means am I 100% in telling sexes or even the number. I have had one incognito in there.
There seems to be a slightly different sound in most though, not just the speed. Boys tend to be more tick,tick,tick and girls more of a tick,toc,tick,toc. I'm sure that's as clear as mud. LOL
I usually listen at night when we lay down. Its almost as sweet as the kicks I get when they are laying against me.