I'd try a quick test with the 16 bit audio setting first. It's possible your camera is defaulting to 12 bit and your editing software isn't correctly converting it to the PC's native 16 bit.
If it's still tinny, perhaps you're got an audio compressor/decompressor that's got a bug in it. DV audio from the camera has no compression (LPCM), but by the time you're done your editing and are ready to burn the DVD, the audio may have been compressed, decompressed, bit rate changed back and forth and recompressed, thereby multiplying the errors.
Try re-installing Windows Mediaplayer and DirectX too. That's been known to fix subtle audio and video problems.
Fav quote - "Experience is whatcha don't get 'till ya don't need it no more."
System - Athlon 1.4GHz, Win98, Hauppauge PVR250 receiver and compressor.
Software -Magix Movie Edit Pro 10, Nero 6 + NeroVision Express, Moho 4.61, PSP 8.1, Bryce, Quicktime 6.52 pro, Goldwave 5, DVD-Lab.
Cameras - Panasonic GS9, Canon ES8400V, Canon EOS D20 and Canon A70