Making audio react to opacity

Hi all, thank you for reading this.

I'm a little noob with expressions and now I just realized i REALLY a noob.

I'm trying to make the audio level of a sfx go up and down along with one layer's opacity : when the opacity go down, the audio goes up, and vice versa.

I linked the audio level to the opacity, plus a linear expression :

temp = thisComp.layer("fond anime.avi").transform.opacity;
[temp, temp];
S=linear(temp,0,100,-48,0);
[S,S]

But the audio levels aren't changing.
Any ideas ? Please.. I can do your laundry :)