My first steps would be:
- Collect and provide Plexamp logs from the current configuration after attempting playback.
- Connect speakers or a headset to the 3.5mm port on the Pi and select the bcm headset device as the playback device; see if it works properly in that configuration. (Just as a troubleshooting step.)
Before doing anything else though, ensure you have a Player connected in the Players dialog by clicking the player selection icon at the top-right of the web UI (if you’re using that):
Weird things can happen if you don’t have one connected and you attempt playback.