Reply to weird mapping issue with s4

Hey guys, sorry if this is the wrong community ... but quick question:

I decided to run my apc40 along side my s4 to use the buttons for all 8 cue points, etc. However, when I use both together... my headphones stop working. No more cueing. No apc, no problems. I've checked twice, and there doesn't seem to be any midi conflicts. Anybody have any guesses as to what might be happening?

Thanks in advance for the help!

Dexx