The really weird thing is that it's been working fine routed through a Pioneer receiver for about 4-5 months, and only stopped having any sound a few days ago. Factory reset doesn't do anything. Connecting the HDMI directly from the Mi Box to the TV seemed to solve this problem, however I'd much rather get the nice sound I can get through my surround system.
(Every other device connected through the receiver works perfectly, and the sound problems on the Mi Box appear to be universal; no apps were producing any sound.)
Any idea what could be causing this issue to appear out of nowhere?
Have you checked the receiver though? Maybe the HDMI port is defective, try another one?