VSL (Virtual StudioLive for computer) as well as StudioLive Remote (for iPad) are missing some functions of the board. The gains, aux outs and aux ins can only be controlled by the knobs on the board, while subgroups and mains are part of remote control. That's the design of the classic boards...

The volume knobs/faders of aux sends are simply missing, but there is a better way to control the auxes than by the tiny little bars in "Overview". The tiny bars are there for a fast orientation, and you can also move the bars, but that is not the only way to do it. I am not sure any longer where to find the individual auxes: It's the the "Aux" or "Masters" button above the Subgroup and Main section. You will find bigger displays there like the ones on iPad. If you want to have a look at the manuals, this is the page for you: https://www.presonus.com/support/downloads/StudioLive-24.4.2.

If I understand you right you have extended your computer connections (keyboard, monitor, mouse) to the booth, therefore I would suggest to place your CD-player there also. You only need a small (analog) mixer with two line inputs and balanced stereo output. Get two long cables, connect the outputs to "Aux In A" (stereo input) and set the volume knob to about half of the scale. Now you can control the volume of the CDs on the "external" mixer. The "Aux Ins" are individually adjustable on all auxes and the mains. You can also add a microphone to the additional mixer for talkback purposes.

Hope this helps.