I thought you might be about to ask that ; ) Yes there is... you want to use a 'linear interpolation' expression. It maps one range of values to another. You can find it under the expression language reference menu - the black triangle in the white circle just to the right of the pickwhip. Go to interpolation > and the second entry. You'll get:
linear(t, tMin, tMax, value1, value2)
in common written form: "As 'something' goes from min value to max value, I want to go from value 1 to value 2"
You need to replace these placeholder values. So look at your audio levels in the graph editor and decide on the min and max values you want to use. From your video maybe 5 min and 25 max.
So here's your expression:
audioAmp = thisComp.layer("Audio Amplitude").effect("Both Channels")("Slider"); linear(audioAmp, 5, 25, 40, 100)
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