Sean, I did say "assuming that you have the correct HDMI connectionS". As Ken pointed out, your clarification on your connections establishes that you don't; you have no HDMI(or even DVI)connection from your cable box at all. Although the audio carried by the optical cable is digital, as is the video which a HDMI cable carries, that doesn't mean that that the digital audio can simply be added to the digital video and both be carried by HDMI. The formats are different and the digital audio would have to be transcoded into a form which could use HDMI. I recall there are a couple separate units to do this, but don't know that any receiver does. So, unless your cable box has an HDMI output(and chess and Royce indicate that it may not; DVI doesn't carry audio), you won't get sound on the HDMI input on your TV. You'd have to use one of the other inputs for sound.
What may have misled you is the fact that your 74TXVi does "convert" the composite video input to a digital output that HDMI can use and a picture resulted. You may have thought that something similar applied to the audio, but it doesn't. Incidentally, if you do mean the single cable composite video rather than the three cable component video, that's the poorest of the commonly used connections. The fact that it's carried on HDMI doesn't improve its quality in any way, and it's still at the composite video level. Better would be to use a DVI/HDMI cable out of the box(if in fact it does use a DVI output)into the 74 for video. Alternatively, S-video or component connections would be better if you do have composite now.
