This is my first real issue that I can't seem to diagnose myself.

After starting my game back up tonight the game volume is MUCH higher than it was before, despite the settings remaining the same.
I first noticed when I loaded a save as the sound effect the game makes in the menu when loading into the game made me jump. Then I ran into a group of bandits not long after loading in, the sword impacts and clangs very very loud and I had to play with my headphones off until I could finish the fight.

The important thing is that the settings are the same as they were last night, ingame and also at a system level.
Anybody have any idea what could be causing this?

I know this may sound pretty obvious, but does your headphones have volume control? Easily overlooked.

They do have a volume control but it's always set to Max as they are very quiet set as anything else.
All other system sounds are the same, including other games it's just Witcher 3 that is different

music - sound effects - dubbing? all?

you can turn it ingame down, without touching your system sound preference pane
or you can use your preference pane to adjust just the game output level inside your windows mixer while its running

There are general issues with audio in this game.
I found the issue in mine. Thing is - my Sound settings in Control Panel in Windows are set for 4 speakers despite the fact that I use headphones for years.
Seems like your game jump from one "speaker" to another to make realistic experience, if you have good speakers that's it.
It did in my game with dialogues. then I went and limited "speakers" to 2 and problem solved.