Hi there, to switch off on-board BT on RPi3 you can use the dtoverlay=pi3-disable-bt overlay, below are some helpful links:
https://www.balena.io/docs/reference/OS/advanced/
https://www.raspberrypi.org/documentation/configuration/uart.md
https://blog.sleeplessbeastie.eu/2018/12/31/how-to-disable-onboard-wifi-and-bluetooth-on-raspberry-pi-3/
Once that is done, the external dongle should be picked up by default by balenaSound.
You could also just disable BT and use a protocol running over Wi-Fi, such as AirPlay, depending on your desktop/mobile hardware support.