Your requirements...
Socket 939 - plenty of choices
no onboard video - AFAIK only 2 choices for mATX, assuming you need PCI-E 16x
mATX form factor - plenty of choices
onboard sound - all boards
SPDIF out - rare
overclocking - not very rare, but rare to be "good" and with voltages
All the following are mATX socket 939 boards with onboard sound and at least minor overclocking. All
should be stable and perform nearly identical to each other. Your choice would be based on how much you value various aspects of your "wish list" and availability. I'll give brief notes on features. If I mention "fan" that means chipset has a fan on it, a potential source for noise. "Minor" overclocking means some RAM and HTT adjustments. "Good" overclocking means having voltages and I've heard reports of 300MHz+ HTT. "Average" overclocking means maybe voltages and maybe some middling HTT speeds (240-260MHz). Almost all of these boards have one PCI-E 16x, one PCI-E 1x and two PCI. First two boards don't have onboard video, the rest do but you can definately use a PCI-E video card in them just fine. IMO onboard video doesn't matter, just ignore it.
Foxconn NF4K8MC-EKRS
AFAIK the first (out of two) mATX socket 939 boards without integrated video. Uses Nforce4 chipset. Minor overclocking, Firewire, fan. Only 2 RAM slots. $73 shipped.
eVGA 131-K8-NF44-AX
AFAIK the second (out of two) mATX socket 939 boards without integrated video. Also, the only mATX board with SLI. Yes, SLI. Average overclocking, Firewire, fan,l SPDIF out (optical and coaxial), only 1 PCI (tough to run dual tuners). $82 shipped.
Foxconn 6150K8MA-8EKRS
Average overclocking. 3x PCI slots but no PCI-E 1x. Firewire. $73 shipped.
DFI RS482 INFINITY
Good overclocking, Firewire, SPDIF out (optical). $90 shipped but OOS.
EPoX EP-9GF6100-M
Good overclocking, fan. $69 shipped but OOS.
BIOSTAR GEFORCE 6100-M9
Average overclocking. $66 shipped. Note that this is similar to the "Tforce" version below, but overclocking will be not as good.
BIOSTAR TForce6100-939
Good overclocking. $76 shipped but OOS. I have this board and it is very good. Runs my Opteron 144 at 2.7GHz on a 300MHz HTT just fine, may be able to do more but the fanless chipset heatsink gets pretty hot which worries me.
Here's what sets these boards apart, IMO.
SPDIF out - DFI and eVGA
Good overclocking - DFI, EPoX and Biostar
SLI - eVGA