Yeah this thread is kinda off from the original topic 
did you comment this line on the boot/config.txt
#dtparam=audio=on
I had the same issue as yours and there’s no sound from the line out of the pirate audio dac hat. I checked the logs and there is just one error on the /dev/ttyUSB0 which I think it’s OK to have that.
The plexamp was using the default audio of the rpi zero. After I commented the line above, I got it to play on the DAC.
I also discovered something on plexamp on raspberry pi:
Jun 14 21:57:17 raspberrypi node[449]: debug: [TREBLE] BASS: Starting up.
Jun 14 21:57:17 raspberrypi node[449]: debug: [TREBLE] Device 1: Default - enabled default - sample rates:
Jun 14 21:57:17 raspberrypi node[449]: debug: [TREBLE] Device 2: snd_rpi_hifiberry_dac: HifiBerry DAC HiFi pcm5102a-hifi-0 - enabled - sample rates:
Jun 14 21:57:17 raspberrypi node[449]: debug: [TREBLE] BASS: Using plugin path '/home/pi/plexamp/server/node_modules/treble/build/Release/'.
Jun 14 21:57:17 raspberrypi node[449]: debug: [TREBLE] BASS: Reloading ourselves for symbol visibility.
Jun 14 21:57:17 raspberrypi node[449]: debug: [TREBLE] BASS: Version 2.4.13.8
Jun 14 21:57:17 raspberrypi node[449]: debug: [TREBLE] BASS: Mixer version 2.4.9.2
Jun 14 21:57:17 raspberrypi node[449]: debug: [TREBLE] BASS: Going to try to open device with sample rate of 44100.
Jun 14 21:57:17 raspberrypi node[449]: debug: [TREBLE] BASS: Device opened and sample rate is 44100, latency is 34ms (minimum buffer: 14ms).
It seems that the sampling rate is set to 44.1KHz. The DAC can sample up to 192KHz and it seems that the plexamp for RPi doesn’t seem to go beyond 44.1KHz.
