Is there a way to control the JACK Output volume via terminal or get a PulseAudio Input?

Update July 21, 2019

From a professional music website for Linux:

• Jack Master Volume?

Sorry for posting non-ardour question but I need your guidance. When
Jack takes control over audio, many apps, including flashplayer, act
very loud i.e in VLC I have volume set to 5%. I would like to control
the master volume like in alsamixer instead of setting levels for each
app individually. How can I do that?

There are two answers posted and this one is the most helpful:

there is no such concept. JACK is designed for pro-audio and music
creation workflows. Its not a desktop sound server, even though some
of use it in that way. If you need such a thing, you will need to
route all your JACK clients via a mixing client which could be
something as simply as JackMix or as complex as Ardour. JACK itself
does not provide this facility, and its totally out of the scope of
its design.

The simplest solution then is to install JackMix:

“Ever struggled with a number of jack applications on your desktop everyone using its own master volume-fader but not one common place for all the volumes directly accessible?”

“The solution to your problem is JackMix, a mixer app for jack that looks exactly like the mixer you would use if you had to connect your analog equipment.”

There are other applications for Jack which you can find listed here.

Original answer

I must confess to be musically-challenged but I think this script is where you are heading:

You can get a list of all sinks with pacmd list-sinks, and set the volume with pacmd set-sink-volume`, so you need to do something like

VOLUME='+5%'  
for SINK in `pacmd list-sinks | grep 'index:' | cut -b12-`  
do  
  pacmd set-sink-volume $SINK $VOLUME  
done  

where $VOLUME can be absolute (150%) or relative (+5%, -5%), and
possibly other formats, too.

Most window managers can be configured to launch scripts or programs,
complete with arguments, when you press keys. That’s the best method,
but if your WM doesn’t, there are tools like xbindkeys. So you can
customize in any way you want.

Note that Pulseaudio will start using hardware mixers if the sink
volume goes over 100%, and that can distort the sound.

Also note that Pulseaudio allows to set the volume for each
application (“audio stream”) with pamcd set-sink-input-volume. You can
list them with pacmd list-sink-inputs and set them in a similar way.

That allows you have the sink volumes at a fixed level so they are
about equal, without using hardware mixers, and when you switch sinks,
it will automatically have the “right” volume. That’s the setup I
prefer.