On iOS (and maybe on Android devices), selecting BT microphone allows using BT speakers but might not allow using connected USB speakers.
If a user decides to change from built-in microphone to BT microphone, and the audio was routed to a USB speaker, audio will be rerouted, most probably to BT speakers.
There is currently no way to allow a web site to know which combo can work.
There is also currently no way to allow a web site to know whether audio rerouting happened.
It would be good to discuss what could be done in those circumstances.