Dang alright that's sad.You cant change the volume of individual sounds in xml.
There is a mod that changes volumes of some sounds, but it can't be done through xml. To my knowledge, you'd need to rip the audio files from the game, then lower them manually in something like audacity, then make them back into unity assets and replace them in a mod. This is the mod that does it, if you want an example to go off of, though I don't have any experience with ripping or creating unity assets so you'll have to ask someone else about that.
Dang alright that's sad.
To my knowledge, the way the audio works is that audio comes out of an audio source (if you look at the xmls, each sound has one). These are a unity thing that have a volume built in, as well as some other parameters I'm unfamiliar with. You can, at the very least edit the relative amplitude, if not the peak amplitude and it will have an effect in the game (I tried this out a few days ago and confirmed at least relative amplitude change, meaning you could leave 0.01 seconds at the current peak amplitude and have the rest be really quiet). This is the same reason you'll see a lot of custom entities that have audio as a core of their identity using the screamerVO as their audio source, it's loud. So that's partially true, but you can do something with just audio editing.I have successfully ripped sounds from a mod (specifically Bdub's helicopter sounds) and they seem loud when played on VLC. In-game they sound so tame.
I don't think editing the WAV file and reimporting will fix it. It has something to do in the game instead.