Very puzzled at lack of audio playback on TV

I wouldn’t consider myself a total Newbie - but!
Last week I had a Bios Crash - overheated system - This may be the cause as I can’t remeber if the problem started before or after the crash (I think that it started ocurring BEFORE)
I have a pioneer DV-R 107D - latest f-ware, 1 Gb memory 2.4 ghz Athlon, XP SP2
using Nero 7 to Convert AVI to MPEG2 & burn - video & Audio on PC when plying the disk - NO Audio on my non-PC DVD
Use CopyToDVD to burn VOBs - Same problem
use mainconcept encoder to create MPeG2s, then Tempgenc3 to cretae VOBs - Same Problem
Burn an ISO (not created by me, but d-loaded) using Nero - Picture AND Audio on TV. If I burn a DVD using NERO 7 from an IMG file - video & Audio on both PC & TV.
Audio CDs via Nero burn perfectly - Audio on both pC & TV
I have also re-installed the OS & Nero - and the problem persists
I can’t think that it could be CODEC related
Any ideas gratefully received.
Thanx