Andrew:
If you unplug/replug the HDMI cable does the audio reset?
If so, then you have a Hot Plug/EDID issue (not uncommon), use our HDM-AIO2 to turn off Hot Plug when you turn off the TV (the HDM-AIO2 does require a triggering relay, if you do not have a control system with one use also our CS-IRKITCCUS for the relay).