I've installed the flatpak package on Archlinux. I can't set with JACK.Flatpak version is 1.10.2-1
JACK does not support Flatpak. In some cases it may work but fundamentally JACK is not designed to support Flatpak.
I recommend using ALSA if you are not using other JACK programs at the same time as jammr. Remember to disable the sound card in PulseAudio so there is no conflict (launch pavucontrol, go to Configuration, and disable the sound card).
In the longer term Linux audio is migrating from PulseAudio and JACK to PipeWire (https://wiki.archlinux.org/index.php/Pipewire
). I haven't tested PipeWire yet so I can't give specific advice but future versions of the jammr flatpak will support ALSA and PipeWire. PipeWire has a JACK-compatible interface so in theory the jammr flatpak might already work with PipeWire today.
If you need JACK, here are some more details:
The jammr flatpak is built with jack2 1.9.14. If your system uses the same version of jackd then it will work. There is more discussion here: https://github.com/flatpak/flatpak/issues/3622
The reason why JACK does not support Flatpak is because applications running inside a Flatpak container may be built against a different version of the JACK library than the jackd running on your system. JACK does not support multiple versions. It assumes all components of the system are using the same JACK version.
You can either build jack2 1.9.14 from source so it is compatible with the jammr flatpak or you can build jammr from source without Flatpak. You can build jammr as follows:
$ git clone https://github.com/wahjam/wahjam.git
$ git checkout 1.3.0
$ qmake CONFIG+=jammr CONFIG+=qtclient
The dependencies are libogg, libvorbis, PortAudio, PortMIDI, Qt 5, and qtkeychain. The jammr binary is located in qtclient/jammr after the build finishes.