I have a sound marker AMBrumbleshake, which is quite loud and can be heard a long distance away from the object it is making sound for, is there a way I can turn down the volume and/or reduce the distance it is heard from? It is too overpowering. I am guessing there is something in its SNOR to do this but I've tried a few settings and nothing changes it.

I think you have to actually reduce the volume of the sound file that is played. You can do this in any sound editor. If this sound is used elsewhere, just give it a unique name.

It's what I do with mods that have waterfalls and the waterfalls are too loud.

The form you're looking for is "AMBRumbleShakeSD" but there is no way, as fas as I have noticed, to change the loudness. You'll have to modify the wav sound files themselves.