Thought it may be a problem with Windows Media Player (I use v7.01 - I have W98SE) but I uninstalled and tried both earlier (v6.4) and later (V9.0) versions with no change.
I did notice that the sound was 'breaking up/distorted) for a few days before, but now sound is ok and video has gone !!!
Some video files play ok - maybe 1 in 5 or so - and I've no problems playing mp3 or wav files.
The problem is NOT just WMP either. The same thing is happening with DivX player (v2.5.5)
All seems ok in device manager and no errors otherwise.
Any ideas ?