Volume controller works on key pad and key pad made up by carbon which disappear after using it and it breaks circuit which is unable to complete your command. In this situation sometime is works and sometimes not and one day it will fully stop.Its really irritate.
Solution-Simply open Sound track button and try to clean it very lightly because if your are cleaning it hardly all the carbon pasted over there will remove it will permanently damage.
After light clean if its still not work simply replace it with new one. Its very easy to change with the help of heat soldering.
http://i00.i.aliimg.com/wsphoto/v0/1038022484/10pcs-font-b-PCB-b-font-Volume-font-b-potentiometer-b-font-board-for-ALPS-09.jpg