I know it's a silly piece of code here. But the gist of it is that I want to create a sound with volume 0, and then increase that volume. The example above will not work. It works if I do it the other way around, so it seems I have to have 100 volume in snd_loop. Why is that?

When you give 0 for any of the params in snd_tune the corresponding value won't ba changed at all. So the semantic of 0 is "don't change the current value" in this special case. Try beginning with a volume of 1 instead of 0. This will do the same, as a sound with volume 0 won't be heard anyways.

I tried that but it wouldnt work either. 2 would work however. Anyway, Im using media_loop instead. It works with volume 0.