For this you'll need to find any given workshop sound mod and you'll need to subscribe to it and launch your game so it downloads.

After it downloads quit the game.

Navigate to your
C:\Program Files(x86)\Steam\steamapps\common\insurgency2\insurgency\custom\workshop
and find your downloaded file.

It looks intimidating at first but if its name should match the workshop name. So for example when looking for the my "Battlefield 2142" mod I would navigate to that folder and search for it (easier if you only have a few mods downloaded).

Once you've found it continue to the next section.

With your sound file selected (remember it should be a .vpk) you need to cut it (ctrl+x)

Paste it (ctrl+v) into the "custom" folder that's right before the workshop folder.
Paste here: C:\Program Files (x86)\Steam\steamapps\common\insurgency2\insurgency\custom

That's it. That's all you have to do and it should play automatically when you launch the game.

DISCLAIMER: It only works with main menu music. Unfortunately you'll still have to use snd_restart for ingame stuff, I suggest binding it to a key. You can do this by simply typing
bind "KEY" "snd_restart" so everything you press KEY the audio restarts.
I've been told that by doing this the audio mods do not require snd_restart. Feel free to follow this guide for all your audio mods.