This can be done with mencoder, which is part of mplayer (www.mplayerhq.nu). The following command usually gets it done for me. Read the documentation for mplayer for more information on the settings:
mencoder -ovc copy -audiofile [whatever.wav] -output [outputfile.avi] [url=http://mf://%5bwhateverfiles.tga]]mf://[whateverfiles.tga][/url]
So if you have a directory of targa files or something from your playblast and an audio file called hpvslameaudio.wav you could cd to the directory with your images and do mencoder -ovc copy -audiofile hpvslameaudio.wav -output playblast.avi mf://*.tga
and you'd end up with a playblast.avi file with audio.