I use Linux Mint 20 and have successfully installed JACK which thanks to Cadence works well in conjuction with PulseAudio.
Subsequently I was so pleased with myself when I manged how to get my bluetooth headphones working thanks to posts on this forum.
I tend to use the CLI programs 'alsamixer' and 'pulsemixer' to control all my volume needs.
So my bluetooth headphones ( yes I have 3 different brands - all work ) well with Audacity, Celluliod, Zoom, - all the 'usual' audio visual programs BUT ....OH NO ....DISASTER.... I can't get them to work with:-

• Frescobaldi ( front end for Lilypond)
• Rosgarden ( My favourite DAW)

both of which rely on MIDI sounds provided by QSynth
Oh yes these programs work well through the laptop speakers or WIRED headphones but not bluetooth.
Could anyone point me in the correct direction?

That is strange because the point of JACK is to separate the connections from the software, in other words, if JACK recognizes the BT phones, ALL JACK software should be able to recognize them, too. Maybe the routing is wrong. Maybe those softwares default to pulseaudio, not JACK, or vice versa.

BTW, you don't necessarily need pulseaudio if you use jackaudio, or you can use pulseaudio as a jack plugin only.

Could you elaborate on that please ?

Most of my programs 'want'to use PA which is why I was pleased that PA and JACK can work together.

Pulse and JACK can coexist in 2 different ways.
I can't get much more detailed than that, but check out
https://wiki.archlinux.org/index.php...Connection_Kit (search for pulse)
and
https://wiki.archlinux.org/index.php/PulseAudio (search fro jack)

But that's not your main problem atm, which is why I posted is as "BTW".