OK, now I got it.
Let me give the long 'story' for other readers' info.
The mobo is GIGABYTE GA-M51GM-S2G. Audio I/O @ back panel has 6 jacks, labelled :
1-center/subwoofer speaker out;
2-surround speaker out(rear speaker out);
3-side speaker out;
4-line in;
5-line out (front speaker out);
6-mic in.
In WinXP, I connected spearker to 2-surround speaker out(rear speaker out). Sound was available.
With speakers on 2, Vista was 'silent' dispite installing various vista drivers.
On 20-04-2007, with speakers still on 2, I plugged headphone on 5-line out (front speaker out) and heard music from the audio CD playing.
Un-plug headphone moved speaker cable to 5, music produced on speakers !
Shutdown reboot individually into WinXP & Vista, sound is available on both OS systems.
I do not understand why XP could use 2, when Vista 'refused'; and both 'agreed' to use 5.
Anyway it is upto Gigabyte and Realtek to sort it out.