Check "Show:" at the bottom of the Playback Tab in pavucontrol?
I see it both in Playback and Recording and I can control it in both also.
But not when Show: is set to "Applications"
     (Same versions of OS and pavucontrol as you have)

I see it here:
root2:~/>pactl list sink-inputs
Sink Input #94
 	Driver: module-loopback.c
 	Owner Module: 33
 	Client: n/a
 	Sink: 2
 	Sample Specification: s16le 2ch 44099Hz
 	Channel Map: front-left,front-right
 	Format: pcm, format.sample_format = "\"s16le\""  format.rate = "44100" 
format.channels = "2"  format.channel_map = "\"front-left,front-right\""
 	Corked: no
 	Mute: no
 	Volume: front-left: 60893 /  93% / -1.91 dB,   front-right: 60893 /  93% 
/ -1.91 dB
 	        balance 0.00
 	Buffer Latency: 7324 usec
 	Sink Latency: 13960 usec
 	Resample method: speex-float-1
 	Properties:
 		media.role = "abstract"
 		module-stream-restore.id = "sink-input-by-media-role:abstract"
 		media.name = "Loopback from USB Headset H540 Analog Stereo"

So you could control it as a "sink-input" with
     pactl set-sink-input-volume 94 40000    (normal (100%) is 0x10000 or 65536)
     pactl set-sink-input-volume 94 80%
     pactl set-sink-input-volume 94 0.7

Search for set-sink-input-volume in the pactl man page.


On Tue, 16 Jun 2020, gregrwm wrote:

> hmm:
> 
> >$ pactl load-module module-loopback latency_msec=1
> >27
> >$ pactl load-module module-loopback latency_msec=1
> >28
> 
> each of the above was followed by a quick flash on the Playback tab of pavucontrol, like some device appeared but disappeared again
> far too quickly to see what it was.
> 
> >$ pactl list
> >...
> > Module #27
> >   Name: module-loopback
> >   Argument: latency_msec=1
> >   Usage counter: n/a
> >   Properties:
> >      module.author = "Pierre-Louis Bossart"
> >      module.description = "Loopback from source to sink"
> >      module.version = "13.99.1"
> > Module #28
> >   Name: module-loopback
> >   Argument: latency_msec=1 Usage counter: n/a
> >   Properties:
> >      module.author = "Pierre-Louis Bossart"
> >      module.description = "Loopback from source to sink"
> >      module.version = "13.99.1"
> >...
> 
> and yes i do have "sideband" now from mic into earcups with a tiny delay.  thanks!  but the quick flash and immediate disappearance
> of the device in pavucontrol seems bizarre to me.  actually the volume of it is about perfect, but if i were to want to observe or
> adjust it at all...any idea?
> 
> (focal ubuntu pavucontrol 4.0-1build1)
> 
> 
> On Tue, Jun 16, 2020 at 4:24 PM gerry <gsker at skerbitz.org> wrote:
>       I just did this:
>
>       https://askubuntu.com/questions/123798/how-to-hear-my-voice-in-speakers-with-a-mic
>
>       In fact the only part of that I did was
>          pactl load-module module-loopback latency_msec=1
>
>       And immediately I had another device that showed up in pavucontrol in my
>       Playback tab called
>           "Loopback from USB Headset H540 Analog Stereo"
>
>       And now I can hear myself in the headphones.
> 
> 
>