I want to play a metronome audio tick out of the built-in headphone port, while playing an audio stream out of a USB soundcard. How can I do this?
JACK is running the USB soundcard, and Pulseaudio is probably the thing that is in charge of the built-in soundcard*.
My first thought is to use both JACK and Pulseaudio in tandem, using the JACK sink for pulseaudio input. Routing the metronome to the Pulseaudio JACK source makes the JACK source appear in pavucontrol under the 'Input Devices' tab - the mixer shows it is receiving the metronome signal. But not matter what I can do I can't get this to actually be output by pulseaudio at any point.
How can I send the signal from JACK source to the built-in headphone port while still using the USB soundcard?
(* Tested this by using the 'Test Sound' function in Linux Mint's Sound manager - plays a test tone happily out of the headphones, which suggests this should be possible!)