Q: I'm using a DVI-out to HDMI-in cable from my HP mediacenter to my Sony TV/ monitor, and getting no sound. Do I need a separate audio cable?
A: The reason you're not getting any sound is because the DVI connection doesn't transfer audio, just video.
HDMI transfers video at the same quality levels as DVI, but the added advantage of HDMI is that it also carries sound through the one cable. Since the output on your HP mediacentre is DVI, you'll need to also connect an audio cable (the red/white or left/right stereo kind).