cannot hear sound outside Ardour

Hi all,
until now I made my tracks using headphones. Since some days I’ve got a pair of monitors. Now I have a problem. When I’m in Ardour there’s no kind of problem, I hear the sound as always.
But. Outside Ardour I don’t hear any sound coming from the monitors.
Sometimes I use Audacity to put different tracks together, though I don’t hear any sound in the monitors.
Also if I want to listen to a tune I have in the computer using the monitors, I can’t. No sound at all.
Even though in the audio settings I select Fast Track Pro instead Speaker - Internal Audio, there’s no chance to hear anything.
I’ve also tried to start Ardour with a track ready to play, at the same time I’ve started Audacity with another track: I hear the one in Ardou, I don’t the one in Audacity.
What might be? And, is there a way to let the system using the external sound card once I turn it on, excluding the on board one?
I have Ubuntu 16.04 64bit Thank you.

While Ardour is running, other applications cannot use the same audio interface. If you use JACK, while JACK is running, non-JACK applications cannot use the same audio interface. This is a feature, not a bug. If you want to mix the audio output of several applications in a pro-audio/music creation context, use JACK and apps that use JACK.

No, that was only a try. I’ve tried starting Qjackctl and Audacity, but I cannot hearing any sound.
Even if in Audacity ALSA and fast track pro are selected.
Same stuff if I’m using Rythmbox to listen to the sound. If I start Qjackctl and try to listen to a wav or flac file, I cannot hear any sound.
I don’t understand what might be the problem.

i mentioned Rythmbox also if it’s not an application using jack, clearly.

If you have jack running you can not hear sound from Audacity unless you tell it to use Jack also. This can be done by clicking on a button on the mid upper part of the audacity user interface. On my computer the button is underneath the microphone icon and it reads: ALSA and when I click on it I can select between ALSA and Jack.

I’ve got it. This time it worked.
The first time I’ve chose Jack but without Jack running, I guess, that’s why it crashed.
Since when I chose Jack in the dropdown list where’s also ALSA, in the other dropdown lists on the side it appears system, no sound card is mentioned, I thought there was a problem with that.
But now I’ve started Jack, started Audacity and chose Jack. In this way it works. I have to remember it.
Thank you.