How does the DAC on the 52j nano board chip output audio after mixing left and right channels?

I am using a 52j nano board. I want to use the mono DAC inside the chip, and I need to mix the left and right channels from Bluetooth and output them to the chip’s built-in DAC. I am using the a2dp_multi_connect example, but Bluetooth seems to only play the left channel; the right channel is not coming out. Could you please help me look into this and solve the problem? Thank you!