sound bard is middle of top row(but familiarise yourself with the others)...

if it were the coin door buttons.. wouldnt you see bargraph volume ADJ? (i thought they all did this (wpc))

sounds data 'ish to me

check from where the volume adj data is coming from on the soundbaord and see if everything looks ok.
or i reckon C18 could be a dry joint culprit as an early bet !

regards