Firstly have you tried updating your sound drivers and codecs for the DVD software?
Secondly, do you have onboard sound? I have and although for most of the time it's good. Sometimes when the CPU get higher load (eg when starting another program) it starts to stutter because onboard sound is more dependent on the CPU than if you have a sound card which will process the sound and take some strain off the CPU. Decoding DVD can be fairly CPU intensive, as it decodes images and sound at once and has to synchronise them. Don't forget that IE and MSN have to connect to the internet and therefore use more CPU power when sending/recieving data. All I can suggest for this is that you close background programs you don't need, you might be able to set sound as higher priority, but I don't now how to, and it may have an effect on other things.