You need to specify the x value to be the current value.
Your expression could look like and work:
value + [0,pickwhipped slider]
thank you for the answer. it almost works - only at the very first keyframe the upper head layer jumps upwards some 500px, behind the comp boundaries and then at the 2nd keyframe it comes back to its original position and is behaving as i want it to. I also added *-3 after pickwhipped slider in the array to make movement more noticeable.
I would also like to sync head's rotation to the audio so that the movement is more organic, not just an up-and-down thing. I pickwhipped head's Rotation property to the slider and rotation is synced but it rotates only clockwise. How to make it randomly rotate both clockwise and counter-clockwise?
You could use a linear() expression to translate the positive values of the slider to both positive and negative.
Or you could simply subtract about half of the value in the expression too.