Thanks @DementedCorn327
There is indeed a volume knob, like you said, and I found where it is.
It is above the left feet rudder pedal in normal cockpit view.
When in normal cockpit view, looking at the left rudder pedal, where the left feet is suppose to be, right above it is a whole series of green lit volume knobs.
On the top left, is the green lit V/U 1 button, which is the channel that is enabled by default. And right under it, is the volume knob at 0% volume, and also muted. So gotta press it to unmute it, and then also turn the volume up. And then it works.
extra optional radio info
Summary
The V/U 1 is basically shown as Comm 1 on the atc interface. The cockpit instrument for radio settings is two instruments above the right feet rudder pedal, and then 1 instrument to the right. There are the V/U 1, V/U 2 and V/U 3 and their radio frequencies. Which can be set by pressing CLR > 6 number frequency > press green triangle above or bellow green frequency number.
And then it sets that radio frequency for either vu 1, 2 or 3.
Since V/U 1 is enabled at the volume knobs, by default. Setting the number to the triangle above the V/U 1 green frequency, will cause it to engage and enable / activate that radio.
The white frequency is the secondary frequency for quick switch.
So clr > frequency# > green triangle above/bellow white frequency.
This sets the secondary frequency for that V/U.
So then to quick switch between primary and secondary frequency (for V/U 1 in this example) is to press the green triangle above the V/U 1 white frequency number. This causes the radio to switch back and forth, so the white frequency number becomes green and the green one becomes the white one. Switching between e.g. 122.000 and 123.000 back and forth.
The bottom right of the radio frequencies is to set the transponder code. Appearantly that is for safety number identity. Not sure how it works. The M1 does not work. But I can change the m3 one, while the checklist says i should change the m1 one.
There are certain emergency transponder number codes, 7700 maybe? No idea.
But this is all completely optional. For me, it works fine just using the interface ATC.
But it would be cool to have ground/tower on V/U1 set to 100% volume and then enable V/U 2 with 25% volume and set it to ATIS frequency.
Gonna try that, helps to get more info, like wind direction and speed.