HDMI provides no better sound quality than coaxial or optical when watching DVD's - they all pass the same Dolby Digital or DTS bitstream. It can provide a digital audio connection for DVD-Audio and SACD (multichannel PCM or even DSD in some cases), which is not possible via coaxial and optical, but if you are talking about using the speakers in a TV that's basically irrelevant.

If you are talking about using a TV for the audio reproduction, delivering analog audio, coaxial audio, optical audio, or HDMI audio will represent very minimal differences in sound quality. I'd use whichever was most convenient. If your TV has DVI, it probably only has analog audio inputs, so use those. I'd still use HDMI-to-DVI for video, though. Just get an adapter and use the cable that came with the 980H for your video.