Connecting additional audio devices to JACK

First, make sure jackd is running. I usually start jack using qjackctl for easy UI controls
Next, find which device you want to connect

   1 $ pactl list | egrep 'alsa.card|alsa.card_name'
   2 
   3 # Example output:
   4 ...
   5 Driver: module-alsa-card.c
   6         alsa.card = "0"
   7         alsa.card_name = "HDA Intel PCH"
   8 Driver: module-alsa-card.c
   9         alsa.card = "1"
  10         alsa.card_name = "HDA NVidia"
  11 ...

Then use alsa_in or alsa_out, depending on whether you want to connect an input or an output device. For example:

   1 alsa_in -d hw:3 -r 4800 -p 512
   2 
   3 # -d Device      : use the alsa.card number you found before for this
   4 # -r Sample Rate : alsa_in can resample when necessary
   5 # -p Period Size : amount of frames per period
   6

Then make sure you connect the new client within the JACK connections panel to one or more outputs, so the audio actually goes somewhere.

Cool (Debian native) Jack compatible software

Name	Description
jack_mixer	Pretty basic mixer, with only custom channels.
lsp-plugins	Interesting plugin set as a whole, with many signal processing tools
lsp-plugins-latency-meter	Round-trip true latency meter
lsp-plugins-impulse-reverb	Convolution Reverb for which you can use any publicly available impulse response
meterbridge	Visual audio meters collection

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Connecting additional audio devices to JACK

Cool (Debian native) Jack compatible software