That is an interesting problem. Here is my guess. Since you you have the problem only when running on battery. I am assuming it has to due with the configuration when running on battery. I am not in front of my powerbook at this time, so I can't look at by settings.
My powerbook does the same thing. It seems like the sound takes longer to initialize when on battery power. I've found that it actually hangs the system for the fraction of a second it takes a sound to play. Strange.. :slant:
After a little more research, I found out why the laptop is exhibiting this behavior when on battery power. For some odd reason, OSX >=10.1.2 shuts off the sound hardware on the laptop when it has been inactive for ~30 seconds. The current workaround is to plug in a set of USB speakers when on battery power.
Here is a link to the official Apple documentation.