While you can import sound into Poser, I would actually recommend a different method. I use a Program called VirtualDub (I actually use one of the Modded versions that supports ACM and variable bitrate MP3). I often mix down my sound in an audio editing program of some sort (Cool Edit, Goldwave, Sonic Fire, etc...) and match the length exactly to the video clip duration. Now, the great thing about Virtuadub is it can work with compressed video like AVI (and depending on the version you use OGM, MKV, MP4) as well as sequential JPGs, and a number of audio formats including ACM, WAV, MP3, and a few others depending on version. So once you have an audio and video track that match it's just a matter of importing them into virtualdub and compressing them out.
Virtuadub can also add effects and chain sequences together and a whole lot more. If anyone wants I can dig up some links to the versions I would recommend and some guides?