COD2 and Sound

Gave my Vista Ultimate computer to my son, after reformat and got it all
running nice. He loves COD2 Multiplayer and after he installed it, he
has sound and everything working fine in Single Player, but in
Multiplayer there is no Sound. Asus P5N-E SLI MoBo, 8800 GTX Graphics
card and On-Board sound via Realtek.

All drivers up to date, especially the Realtek. As he is too far away
from me, for me to go and fiddle and get it right, I need to do it all
by phone. Note I had COD2 on the same machine running fine before reformat.

The 'strange' thing is, he has sound in Multiplayer if he plugs
headphones in (got that trick via Google). Have tried every combination
of 'Running as Administrator' and 'Compatibility to XP SP2', all to no
solution. Also installed latest DX9

Any ideas I can relay will be appreciated, I don't want to spawn another
Vista hater <smile>, as he keeps reminding me how well COD2 ran on his
XP Machine!

Frenchy