The software playback channels have no input gain (volumen) adjustment.
This has to be adjusted in the prior software, DAW or player etc

The faders of the hardware input channels and the faders of the software playback channels are not available for the ARC.
The current list of available functions doesn't include them.

@ramses The article I cited explains my setup. In case you did not read it: I have a recording studio. All musicians and the sound engineer wear phones. Another output is designated as a phone, and it goes to the video camera.

A cheap workaround can be a cable-loopback, then a hardware output is inserted into the signal path of the software playback signal, the ARC can adjust the fader of it when this output is assigned to phones 4.

I used 7-zip to open rmeinstall.exe (the TotalMix installer for a UFX III), and saw these files:

$ ls alF
total 17752
drwxr-xr-x 1 mslinn mslinn     4096 Jan 11 17:46 ./
drwxr-xr-x 1 mslinn mslinn     4096 Jan 11 17:46 ../
-rwxr--r-- 1 mslinn mslinn     3072 Sep 13 08:12 .data*
-rwxr--r-- 1 mslinn mslinn     1024 Sep 13 08:12 .gfids*
-rwxr--r-- 1 mslinn mslinn 36609536 Sep 13 08:12 .rdata*
-rwxr--r-- 1 mslinn mslinn    10240 Sep 13 08:12 .reloc*
drwxr-xr-x 1 mslinn mslinn     4096 Jan 11 17:46 .rsrc/
-rwxr--r-- 1 mslinn mslinn   275456 Sep 13 08:12 .text*
-rwxr--r-- 1 mslinn mslinn    14056 Sep 13 08:12 CERTIFICATE*

I used a hex editor to examine this 37 MB file. I should not post those contents, that might violate RME's trade secrets, but I do see what appears to be the components of menus. Furthermore, .rdata seems to contain model-specific information for various audio interfaces, in a binary format. So the problem might be able to be reduced to providing a compatible, but different, .rdata file.

So it should be possible for RME to allow customers to adjust Software Playback volumes, just by tweaking the menu definition in this file.