position[0] is the pre-expression x value (static value + effect of any keyframes). So this expression:
[position[0],thisComp.layer(“Audio Amplitude”).effect(“Left Channel”)(“Slider”)]
says – use the pre-expression x value for x and the audio amplitude for y. To keep the original y value and affect x only, you’d do it like this:
[thisComp.layer(“Audio Amplitude”).effect(“Left Channel”)(“Slider”),position[1]]
Dan