OK - Win ME and WDM drivers... lets check a few symptoms.
Do the sound drivers install, show up in device manager but when you go to multimedia the playback and record devices are greyed out or not there?
If so, get the 95/98 VXD version of the drivers and install those.
You replaced the motherboard, but it almost sounds like you are using the same drivers as for the old motherboard - that may also be the problem.
If so post the motherboard make and model (and the old one for comparison) - lets go driver hunting for the RIGHT ac97 via drivers!
Edit: Duh - you mentioned the board on the first page -
Here are the current drivers on via arena
There are the 95/98 drivers - It can be a bit of fiddle getting unsigned drivers to work in ME... but often where ac97 wdms fail, the vxd's triumph.
P. S.
When via ac97 first came out one of the regulars here had to hack the inf to get it to install right.... lots of people downloaded that particular "custom" driver :D