Just wanted to know if there's any way to lower the volume or get rid of the continuous sound effects for some spells? The effects for spells like Speed and Night Vision are really loud and annoying compared to the rest of the in-game sounds.

The only thing that might do what you are after is in the Options Menu to Lower either the Master Volume, or the SFX Volume.
I know it is not a perfect solution but at least it should help.

Sadly it looks as though the sound effects for Arx Fatalis are packed in an SFX.pak. I know there are decompilers for .pak files. If you were savy enough, you could download one, decompile the SFX.pak file, edit the spell effect sounds, then recompile and put back into the game. Risky (back up your saves in case you have to reinstall), but might be worth a try if you're up for it.
And I know what you mean, they are loud and annoying, made me not want to use Night Vision. Although, I remember really liking "Repel Undead", had a cool eerie, choral sort of sound.

I don't suppose anyone knows of a .pak file extractor that works with Arx Fatalis files?

Sure:
http://forum.xentax.com/viewtopic.php?f=9&t=1408
(Scroll down to see my post.)

Thanks a lot, that's just what I needed. I was able to put a replacement sound file in a "SFX" directory i created to get it to play that instead of the original. Just need ArxPak to get the right file name to replace. Like Magic_Spell_speed.wav and Magic_Spell_vision.wav for the speed and nightvision loops.
So far I just replaced them with blank files. I might try editing the original sounds to lower the volume later. Or replace them with something different entirely.
Thanks again :)

I just tried lowering the volume of the original sounds and for some reason they won't play in the game once they've been extracted.
Even if I extract the original sounds and just leave them unchanged, whatever sounds I've extracted simply don't play any more. I have to delete them out of the SFX folder so that the originals will play.
There doesn't seem to be any way to actually edit the Arx pak files either. So unless there is some hidden command to allow extracted sound effects to play, its impossible to edit any of them and use them in game. You can make them not play at all, which fixes the annoying sound loops, but that can take away from the gameplay. Plus, some of the incredibly loud noises (like the lightening effect) really should still be audible, just quieter.

Yeah, I had the same result. I thought it was working at first, because I was just replacing with blank files anyway. But when I tried to replace an effect with a different sound it still played nothing.

- extract the sounds
- in the main game directory (or in Libertatis directory, if u play this compatibility mod) create the directory with the same name as is within the extracted files (i.e. SFX)
- find the sound file you need (i.e. Magic_spell_vision.wav or other) within the extracted files. Edit it. This can mean replacing it with zero lenght file if you want the sound effect doing nothing or renaming any wav you want to this name so you have your own effects.
- put the file in the previously mentioned directory

Voilá.