One answer is to do the sounds the other way around.
My SoundStager script would let you load up a single punch sound
and apply it to every contact of the boxing glove and the face,
for instance.

http://market.renderosity.com/mod/bcs/index.php?ViewProduct=57986

(Shameless plug)

Edit: I might add, SoundStager is also meant to serve as an overall 
'track controller' and 'track mixer' for music, Mimic, and contact effects.

=====================

If you're going to do it the other way around, you just need to
read the waveform, which is possible with some practice.

Or use a wave editor to zoom in on the time scale and write down
the exact times where the punch needs to happen.

The hardest part of that is the anticipating, because even after
you know where the contact happens, you need to "walk backwards"
on the Poser timeline to find where the punching move starts.

My python page
My ShareCG freebies