korbaach , while your method is indeed clever and relatively simple, it's limiting because of the sprite dependency.
If you want to fade in and out multiple sounds with independent fade timings, you'll have to either use multiple sprites or many instances of a sprite filtered by instance variables. It's definitely doable but not very practical to setup and trigger.
On the other hand, the rex_audio_helper does exactly that, it plays/stops any sound with any amount of fade. With a single action every time.
It's good to have alternatives and out of the box thinking, but some tools exist to make our life easier and I think that rexrainbow 's work is brilliant and shouldn't be neglected with workarounds.
I am not sure how all this sounds, I mean no disrespect