Hi Freakon, as SetSoundVolume(vol) is a global the only way I think is by either making your own sound wav's for your player or edit the volume of the sound wav in some audio software you my own.
What you need is in this directory:-
\Steam\steamapps\common\Game Guru\Files\audiobank\materials
For one step on grass would be this file 0grassA1.WAV and son on.
For player sounds they are here:-
\Steam\steamapps\common\Game Guru\Files\audiobank\voices
This is for the player sounds like hurt, punch, etc.
I always copy & paste the player audio folder, rename and edit those files then edit the player starter marker properties to just include those files.
I'm don't sure there is a way to change it with lua just yet... but I could be wrong.
Dark Base 900 Pro OJ: Rampage V Ed.10, i7 6950x non-OC, DDR4-3333 64 GB RAM, Win 10/64, Asus Strix GeForce 1080 x 2 SLI mode, Water cooled with an old Victorian cast iron radiator and a industrial leather belt driven fan - That's what you call Steampunked.
Laptop - M17xR3
A new competition running for GG