threshold = 20;
footageComp = comp("FOOTAGES");
audioLev = thisComp.layer("Audio Amplitude").effect("Both Channels")("Slider");
above = null;

for(var t=0;
t<=time;
t+=thisComp.frameDuration)
if(audioLev.valueAtTime(t)>threshold)
{
    above = t;
    t+= 4;
}

seedRandom(timeToFrames(above),true);

n = random(1,footageComp.numLayers+1);
playStartTime = footageComp.layer(n).inPoint;
playEndTime =  footageComp.layer(n).outPoint-thisComp.frameDuration;
playDuration = playEndTime - playStartTime;

if(above)
    linear(time, above, above+playDuration, playStartTime, playEndTime);
else
    0;