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.