Arduino R3 Plus DFPlayer Pro = Barely Any Sound Volume

I am using a DFPlayer Pro sound module driven by an Arduino Uno R3 to play sounds for a Halloween project and I can get only a very faint sound (barely audible) from powered speakers.

I’ve tested with three different sound modules (all DFPlayer Pro originals from DF Robot, no clones).

I have used three different sets of powered speakers, which are plugged into the DAC connectors on the sound module. Same result for all three: very low volume.

Tried both ground connections coming off the Uno. No improvement.

Tried a different Arduino Uno R3 board.

I can hear the music playing very faintly so I know the various .wav files are correct. Checked the sound files sample rates (1441). Tried the same sound files as mp3’s.

When I plug in a small unpowered speaker to the speaker outputs on the module (R+ and L+), I get the same thing: very faint sound.

I understand that the default volume setting on the module is 20 out of 30. I’ve tried unplugging power to everything (to reset them) but no improvement.

This is interesting: I have powered the setup two different ways: desktop USB to the Uno and then to the sound module; and powering the sound module externally with a 5 volt wall wart. I am carefully connecting all grounds. Using this external power arrangement (which I need to do in the final version), I get no sound at all.

By the way, I am not using any serial code or connections. I’m triggering play by simulating a button push using the following code (which seems to work fine):

  pinMode(7,INPUT);  

  delay(100);

  pinMode(7,OUTPUT);

  digitalWrite(7,LOW);

I am at a complete loss. Can anyone suggest why I can’t get this to play at a normal volume?