I'm having a problem just started with rendering to avi in platinum - a 1 hour complex movie, which renders fine except sound is missing. If I try a small loop and render, it comes out fine. I've tried several times rendering the complete movie and with no luck.
I've no idea what's causing this, but have you tried rendering the audio on its own? It might give some indication of whether it's (for example) a resource problem or something specific to the audio. If it's audio specific you could try rendering in smaller sections to see if you can isolate the problem.
Ian, I did another reboot and then the rendering went OK. The sound was OK previously when I rendered a short loop from the same production, but I'll keep your tip in mind if it happens again.