Hmmm...no sound on the right channel? This could be a few things...
-Cable fault. Maybe a solder joint to one of the driver's contacts broke, particularly the diaphragm bias wire. Perhaps there's a fault within the cable itself. If this is the case, it should be relatively easy and affordable to fix if you can get a hold of a new cable.
-The actual driver itself is damaged, perhaps by the stators arcing through the diaphragm and burning a hole in it. I don't know if that would cause it to fall totally silent, but that would be very bad and require a very expensive replacement driver (or finding someone that can rebuild the driver; unfortunately, I don't know who rebuilt the drivers in my Lambda). Should it come to that, you could transplant the Lambda Pro drivers into the Sigma if you wanted to...or you could do what some other people do and make it a Sigma/404, that is, a Sigma with SR-404 drivers, to improve the sound quality even further from stock.