Could you share a testable IPK file that includes an audio file? I will try to test on my RPi. As far as I know, the sound will be outputted through HDMI. So, If your display doesn't have a speaker, you can't hear that.
I tested it on my dev environment. (a RPi4 with webOS OSE 2.6.0, a monitor with speaker)
The app that you sent worked well. I listened to the guitar sound from the app.
So, do you want to change output channel for audio?
Unfortunately, I don't know how to change the audio input/output channel from HDMI to USB sound card. I will ask the Dev Team. But I don't know if I can get an answer.