I don't know about the other's so much but C-Media is pretty decent. I
have that on my Asus mb and used it for a while and had no issues and
the sound quality was ok. AC97 is just a standard and not a sound
chipset although some mb's list their mb as having AC97 for sound.
C-Media is also AC97. I think Realtek is supposed to be ok. My Abit
mb has Realtek ALC650 but I have never tried it out.
Just to complicate things further, the chip is only one relatively
small part of the equation. I've got two systems, both with AC97
sound using C-Media CODECs. The first is an ASRock K7S41GX using a
CMI9739 chip. The second is a Chaintech 7NJL6 using a CMI7961A. The
difference in specs between these two chips is quite small, but the
difference in sound quality is very noticeable. The Chaintech board
has a much cleaner sound, sufficiently better that it's pretty obvious
with my only average-quality computer speakers.
Unfortunately audio is one of those tricky analog things where
subtleties in design can affect performance much more than
specifications.
The only onboard
sound solution that can match an add-on card like an Audigy2 is the
Soundstorm that came on mb's with the Nforce2 chipset.
Virtually none of the nForce2 chipsets use the Soundstorm audio. My
above-mentioned Chaintech board certainly does not. Most of the
nForce1 boards used Soundstorm, but it was mostly dropped for the
nForce2.
But I think all
onboard sound these days is fairly decent for music, mp3's, internet
radio etc. It's when you get into 3D audio in games that they may fall
a bit short. I prefer to use add-on cards myself and have Audigy2 on
one PC and Santa Cruz on the other. It all depends on what your PC
usage is. I'm a gamer and is why I use add-on cards.
I find the integrated audio on my Chaintech board to be quite
sufficient for light-use, but I purchased a PCI sound card for the
ASRock board.