If you need to mix both channel in one output. Please do a little change to the board as following
short the fifth Resistor position in the first line. then move the 2nd Resistor to position 5 in second line.
Found more from page 138 of this topic:
Sorry for the trouble. Just got reply from our tech guy. Did you connect the audio out put with only one channel? for default setting. If you want to use AUDIO out put, you should connect the 4pin socket by "1-2" to left channel and "3-4" to right channel. If you only connect one speaker like "3-4", it will burn the AMP IC. but it will be a easy fix, we can send chips for you.