Clean install of windows?
Any other drivers/patches installed?

btw what version of windows are you using?

Per The Retro Web these boards are known for having bad capacitors, which is something that you have to be ready to deal with when working with hardware from this era. Have you inspected the capacitors? Any chance that they have already been replaced? Does the onboard sound work? What is the history of this motherboard?

This problem doesn't immediately leap out as having a connection to bad caps, but refusing to POST (or does it POST but then not boot into Windows?) with a soundcard added is very odd in my experience, and bad caps can lead to very odd behavior as they often still sort of work.

Try the sound card in another computer.
Determine if is the sound card that is the problem.
I had this with this sound card once and I determined it was a bad sound card.
Not sure how to fix it. But these sound cards were selling for about $10 on eBay at the time.
I bought it used so I really did not care.
But I think I damaged the card once by trying to remove it when the computer was still on.
I forgot to turn off the computer and damaged the sound card.

sorry for late reply.
yes,it's clean install -Win98 sp2
visually,caps look ok
and yes,the sound cards do work and onboard sound works,but seems the original cmedia drivers don't install properly as sound plays intermittently
i'll try win xp later to see if it's hardware problem or win98
thank you for all your replies