(I believe it's the left side) but have same problem - very common with BEATS SOLO .. You have to take apart the left side (?) earpiece VERY CAREFULLY .. Many people wrote in to "you tube" to find the fix - the wire that connects the ear piece to the the rest of the headphones (I believe it's white) comes unsoldered and that's why you can't hear any sound .. You may or may not have to order new headphone covers because there is sticky tape that holds the first piece on - also when you get to the second layer "under a sticker is a screw, so don't forget to take that screw out"!
My problem is that I lost the paperwork on my head phones and can't figure out if they are "BEATS SOLO 1 or 2" - going to attempt to fix mine as soon as I find out which set of "new covers I need" .. Good Luck!