exporting to adobe media encoder

Sesx files aren't your finished video - they contain just the information about the session you created - no audio or video. Yes you can mix down the session to a file, and then export that with the video to Media Encoder, although it's not ideal; what is better in this situation is to use Audition in a 'round trip' situation from within Premiere, and reassemble your final result there. Audition itself can't re-unite video and audio; being an audio editor, it was determined that this was not something it should be able to do (although it could at one stage).

If you get an error using File>Export with Adobe Media Encoder direct from the session, then there are any number of possibilities for why it might fail - not the least of them being the format you were trying to use. I've tried it here using H264 and it works fine, although as I said, I'd normally do something like this starting from Premiere. Can you show us a screengrab of the Export box?