[black_knight]

I don’t know your setup, but Ubuntu uses PulseAudio as default, and it can sometimes take the sound devices hostage. There is a package called pulseaudio-module-jack which will let you output sound from JACK through PulseAudio. Using qjackctl you can see how everything is connected and direct it to the pulse source. The pulse source can be connected, to say your speakers, by loading a loopback module in pactl.

[veli_joza]

There's also Cadence from KXStudio repository[0]. It's a replacement for qjackctrl that can be used to setup pulseaudio-to-jack bridge relatively painlessly.

[0] http://kxstudio.linuxaudio.org/Applications:Cadence