I have a consistent problem with downloaded .rar files that, while attempting to view directly from the PC, shows great video but no sound. Any ideas on how they can be viewed on the PC with sound?

Rar files are NOT videos, but compressed/zipped files.
You need to uncompress them BEFORE you try and play them.

TRy this and let us know if you still have the same issue.

I thought .rar was a compressed format...

Anyway it sounds like a codec problem, try the K-Lite codec pack (search Google for it)

When you have extracted the files from the .rar, try using the utility "AVIcodec". It lets you know what codec for AV files are required.