Manipulating volume on an AudioSource array

public void Audio (){
music.volume = volumesliders[0].value;
sfx[0 - sfx.Length-1].volume = volumesliders[1].value; //NOT FINISHED YET!!!
I always get an ‘index out of range’ error. What do I do?

You can’t put a range of numbers into array brackets. The " - " in there is a minus, so you’re calculating some negative value there, hence the exception.

To do a thing with each element of an array, you have two, or let’s say three options. One is a simple loop:

for(var i = 0; i < sfx.Length; i++)

Then there is foreach loops:
foreach(var source in sfx)
And then there’s Linq:
sfx.ForEach(source => source.DoSomething());
Don’t forget using System.Linq; here.
But, to be honest, there’s another solution - and that is to familiarize yourself with [AudioMixers][1]. They exist for exactly these kind of things; check them out.
[1]: Unity Connect