I just tested this to see what is going on (I hadn't played in a long time). Sadly, the in-game volume sliders do not work (as you already say), and the setsound program does not include CD audio configuration.

Still, you can use DOSBox's mixer command to lower or turn off the volume of the CD audio, as was be suggested in that post you mention (which I did not look for, so maybe I'm reinventing the wheel).

'mixer' It is not an config option, but a command; given at the DOSBox prompt, which can be automated by adding it to the section [autoexec] of the game's config file. In this case the file for the single player configuration is "dosboxSWARS_single.conf".

It is located in the root dir of the game's installation; the same where you found "dosboxSWARS.conf" which contains the common configuration to the different setups that GOG made (single player, settings, server, and client).

You can either give the command with a 0:0 argument, to silence it, or something between 1:1 and 200:200 to set the volume.(*1)

One thing to keep in mind: the default volume is not the maximum. What you are hearing is the volume at 100, not the 200 you can go for, so go with 50 or less if you want to lower the volume considerably.

Another important fact is that the command must be given after the CD image is already mounted or it will not have any effect.

The final result should look like this:

[autoexec]
@echo off
mount C ".."
imgmount d -t iso -fs iso "..\swars\game.ins"
mixer CDAUDIO 0:0
c:
cd SWARS
cls
call play.bat
exit

---

(*1) The two values are for the left and right audio channels, and can be different from one another if you want. But it is not a balance setting. It will actually lower the volume for each channel, and in the case of this game, the audio coming from each channel is not the same (truly stereo).