Don't know if it's related but I had pops and cracks in my audio when setting up 5.1 surround sound in Linux over S/PDIF. I messed with all the stuff mentioned including setting tsched=0 to no avail. The solution was to also set my channels to 6 in alsamixer, even though it was already set to 6 in pulse config. Another tip I read was to try disabling auto-mute in there, but that didn't work for me.