Check the left audio output and driver transistors. Looking at your picture of the innards, I see someone has replaced one of the output transistors with a non-original type. That transistor has a package different from the transistors in the right channel.
PNP power transistors often go open collector in some audio amps, causing distortion and low output. When this fault happens, there is no further damage. (Unlike when shorted transistors take out every other transistor direct coupled with it.)
Bob