Your expression was created using a simple link. The Value of Both Channels typically varies between 0 and about 20, so your layer is going to move to somewhere between 0, 0 and 20, 20.
If you want your layer to move when the audio levels change you have to figure out the minimum and maximum value of the Both Channels slider (pretty easy to do if you use the graph editor and it is set to view the value graph - impossible if you are viewing the speed graph) then you have to decide which axis you want to move by how much realizing that you only have one set of values to choose from. If you assign both x and y to a multiple of the value of Both Channels slider then the layer will only move in a 45º angle. If you just use X or Y you can make the layer move vertically or horizontally. If you want random movement then you will need to use a wiggle expression to drive the expression. To translate the values of the Both Channels slider to usable numbers you will need to use one of the interpolation methods. ease(t, tMin, tMax, value1, value2) is the most common.
If I just wanted the layer to vibrate up and down a maximum of 100 pixels I would set up my expression like this:
t = thisComp.layer("Audio Amplitude").effect("Both Channels")("Slider");
v = linear(t, 0, 18, -50, 50;
[value[0], value[1] + v]
The first line defines the variable t as the value of the Both Channels Slider and we know from the graph editor that those values change from zero to 18 for this audio track.
The second line interprets the value of the slider t which ranges from 0 to 18 to a value between -50 and 50 for a total change of 100 for the value v.
The last line uses the current value for x and add the value of v to the current value of y.
If the audio levels vary between 0 and 18 then the layer will move up and down a maximum of 100 pixels from its original position. If the audio level is 9 the layer will be in its original Y position.
I hope you followed that.
I strongly suggest you spend some time with the User Guide studying expressions. You can't create anything more than a direct link using the Pickwhip in the Parent and Links column.
screenshot of my ae
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