For talking, many of us use an automated lipsync utility, like textPuppet or DAZ's Mimic software.

Music and other audio files can be imported directly into Poser and added to the animation before render-time. It's also possible to edit the rendered animation and add soundtrack elements afterwards, through third-party utilities.

I use VirtualDub and TMPGEnc (both freeware utilities) to edit my videos, adjust the frame-rate, apply filters, and perform other routine tasks.

I don't have it yet, myself, but I can also recommend Wondertouch's particleIllusion software, which creates incredible animated particle-based effects (fire, smoke, magic sparkles, water, rain, snow, etc.), which are very difficult to accomplish with Poser alone. A low-priced special edition is now available, and it's perfect for Poser animators.