Lower volume on one earbud after fixing it (soldering)
Agree with previous poster that you probably have a cold (and highly resistive) solder joint.  Seriously doubt heat damage to the drivers would -only- lower the output.  Heat damage on voice coil/driver that small would distort like crazy.  

You can test this (if space allows) by bypassing the solder joint temporarily) with an external shunt on either side of the soldering point.  Also, look at the DC resistance of both channels.  Cold solder joints are resistive to DC.