I can't seem to find the keybind which allows you to change the volume of your footsteps and stuff, there was a modpack I played where it was bound to the V key but it isn't that by default. Is this just something the modpack added or am I just not finding the keybind?

Which version of DS are you using? If it is the latest 3.5.x versions there isn't a volume control dialog anymore. It was folded into the Minecraft sound menu.

The latest, yes. So how do I make things quieter, is it just the footsteps volume slider? Because that didn't seem to work for me before

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Sorry for the way way way late response. There are two sliders on the Minecraft volume dialog that adjust the general volume of biome sounds as well as footstep sounds. There is also a mod config option called "Individual Sound Configuration" that allows you to tweak individual sounds in the pack.