Don't know about the quality of the support†, or what "decent" onboard sound is, but Supermicro sells a bunch of boards that support 1 or 2 Xeons (and generally also support consumer chips), ECC and have onboard sound. E.g. for the low, low price of $289 this board http://www.newegg.com/Product/Product.aspx?Item=N82E16813182... will support two processors, up to a half terabyte of memory, and has a Realtek ALC888 for 7.1 channels of sound.
†I'm using a one processor chip X9SAE to type this with, with HDMI output with sound included to my receiver and then monitor.
ECC support + decent onboard sound seems like a niche feature combination (at least in the Intel world where ECC support means buying a Xeon and a compatible server/workstation motherboard). Some AMD consumer boards do support ECC if you want to go down that route (e.g. http://www.asus.com/us/Motherboards/M5A97_PLUS/specification... ).
Not sure of your particular usage case but wouldn't it be just as easy to pick up a cheap (or decent) sound card that would meet or beat anything onboard could do? I personally tend to stick with onboard because it does what I need (sound for music and movies) and I use external sound interfaces when I want to mess with anything requiring more sound performance/options (like recording or making my pitiful attempts at producing music).
†I'm using a one processor chip X9SAE to type this with, with HDMI output with sound included to my receiver and then monitor.