… I don’t really know but I have my own ( recent ) experience in this way with the OPL3 board, and I want to share it. After I published the pictures of my FM board, happy with the new sounds ( on audio channels 1-2), I test the channels #3 and #4 and no sound at all!! :sad: then I try some things like to reflow the tin joints, to shorten the flat cable, test_interconnection software and here there was a weird thing happened: when I hit the DO and DO# keys ( in sequence ) a “click” was heard on #3 and #4 channels ( the same thing on #1 and #2 ) then I thought the four channels could be right, then I remembered i recompiled the firmware with the option “AOUT_NG” enabled and this cause some upload errors, so I thought "could it be the recompiled firmware the problem in this case? I thought it due to the following reasons:
I changed channel (to #3 and #4) on Ensamble menu and nothing happens ( Only #1 and #2 working )
I set no audio channel at all and sound continues on #1 and #2
Then I download the compiled hex version ( v1 ), Upload it with MIOS STUDIO and… Crystal and beatiful sound on #3 and #4 channels!! 
But at first some “symptoms” were that similar to what were described (0.28 V on pin 22 C/D; 3.62 V on pin 21 A/B ) in this post:
http://discourse.midibox.org/t/topic/12419
Well, only a curiosity… but may be it could be helpful… I don’t know…