The sound should be decent. As for the firwire port, you can pick up a pci card for this, around $20 to $30 bucks. Just be sure the machine has a few extra pci slots, in case you want to install a sound card or other upgrade at a later date.
I am buying a new Dell 9100 from a reputable seller on Ebay. It is loaded with everything I want except two things. Sound card and IEEE 1394 port. It comes with a intergrated 7.1 dolby sound card. Is that going to be a problem running with some games or programs? Also is it easy to install a IEEE 1394 port?