Unfortunately, supcom is pretty finicky and if you can't use the exact settings pointed out in the audio FAQs by ensuring that only those Windows settings are applied, there isn't much to do.
Make sure that no other drivers or programs related to audio are in use, and try using other speakers or headsets: some of them have their own integrated audio drivers that will add effects or other things which supcom won't like.
Try just plugging the most basic speakers/headphones you can find directly into the motherboard left/right/headphone port (typically it's the green one) to exclude a hardware issue.
The more information you can give, the more help you'll get.