Use this for the scale
scale:
minAudio = 0;
maxAudio = 100;
minStretch = 50;
maxStretch = 80;
audioLev = thisComp.layer("Sound Keys").effect("Sound Keys")("Output 1");
s = linear(audioLev, minAudio, maxAudio, minStretch, maxStretch);
[s,s]
Change the output to what ever output you want to use. The other settings are self explanatory.