I've tried 3 times in the past to run JACK on Ubuntu. Each time I had bricked audio after installing, tried to fix for around 1-2 hours and gave up. My experience with JACK has been horrible.
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.
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.
I've had precisely the opposite experience - for me, JACK is amazing and gives me 40-channels of pro audio over my external audio interfaces (I run a studio with a Linux DAW), but if I put PulseAudio anywhere near this configuration, it dies in a burning pile of poo. I despise PulseAudio and will never use it.