On my Roo 19, A operates the street side speaker and B operates the curbside speaker. C operates the outside speakers. I don't seem to have to crank up the volume for the outside ones, as some above have said.
It's always struck me odd that you would have separate switches for the two inside speakers. If it's "stereo," why wouldn't you just have an indoor switch and an outdoor switch? Are the two outside switches really "stereo," or are there just two speakers? If they're stereo, then the C switch would be switching both the left and right channels on or off simultaneously, while the A and B switches would just be switching one channel each. Weird.
But to your question, I would suspect a loose wire. Other folks have reported loose speaker wires.
1988 Coleman Sequoia - popup (1987-2007)
2012 Roo19 - hybid (2012-2015)
2016 Mini Lite 2503S - tt (2015 - ???)
2011 Traverse LT, 3.6L, FWD
2009 Silverado Ext Cab, 5.3L, 4x4, 3.73
2016 Silverado 2500HD Dbl Cab, 6.0L 4x4, 4.10