user-1: I have been searching all over the internet but still do not know how to do this with After Effects! When a loud beat goes on, the spectrum goes bigger so i did this: 
value + (comp("Music").layer("Audio Amplitude").effect("Both Channels")("Slider"))/ 1
but what i want to do is make it fuzzier or blurry when the beat become loud! How do i do this? Thanks!

user1950: Add a blur effect to the layer, and then use a similar expression to drive the amount of bluriness.
comp("Music").layer("Audio Amplitude").effect("Both Channels")("Slider")*1234

Replace the 1234 constant with whatever value makes sense for the effect–it may have to be a small number like 0.1234 if the effect is too much. One trick I often use is to use the original value as the constant, which means you can adjust the strength of the effect by using the controls for that effect, while still driving it with the expression:
comp("Music").layer("Audio Amplitude").effect("Both Channels")("Slider")*value


-1: <p>I have been searching all over the internet but still do not know how to do this with After Effects! When a loud beat goes on, the spectrum goes bigger so i did this: </p>

<p>value + (comp("Music").layer("Audio Amplitude").effect("Both Channels")("Slider"))/ 1</p>

<p>but what i want to do is make it fuzzier or blurry when the beat become loud! How do i do this? Thanks!</p>
 1950: <p>Add a blur effect to the layer, and then use a similar expression to drive the amount of bluriness.</p>

<pre><code>comp("Music").layer("Audio Amplitude").effect("Both Channels")("Slider")*1234
</code></pre>

<p>Replace the <code>1234</code> constant with whatever value makes sense for the effect–it may have to be a small number like 0.1234 if the effect is too much. One trick I often use is to use the original value as the constant, which means you can adjust the strength of the effect by using the controls for that effect, while still driving it with the expression:</p>

<pre><code>comp("Music").layer("Audio Amplitude").effect("Both Channels")("Slider")*value
</code></pre>
