Here is something that might work:
Looking at the schematic, the signal from the DFPlayer goes in, but has no way of going back to the player.
If you read this topic, @PerryBebbington and a few others explain how signals need a closed loop.
In this case, it looks like your signal is going into the amplifier, but it can't get back out because you switched the IN- signal to a new loop.
Possible fixes: Try connecting the IN- on the amplifier to the GND of the player instead of to the battery so you have a closed loop.
I hope this helps!
@rlsieben