Here's a useful tip to add to your bag of tricks -

Let's say you find yourself and legendary mixer/engineer/producer Bob Clearmountain in front of a room full of audio engineers, Sweetwater Sales Engineers, visiting Apogee dignitaries and a variety of other industry leaders. You're setting up a fairly sophisticated audio system for the purpose of a clinic on high-end Apogee A/D/A converters, and you are unable to get sound through the system. You try a variety of trouble-shooting approaches, check the cables, re-route things, check the powered speakers, fool with the digital clocks and settings, etc., all to no avail. The top 10 things to try in this scenario:

10. Proclaim the whole system to be defective, and change from a product demo to a lecture.
9. Blame someone else in a very loud voice.
8. State in no uncertain terms that you are simply not used to dealing with gear of such inferior quality, and that someone of lesser ability than yourself should be setting this up.
7. Heave the 20-space rack and powered monitors across the room, then calmly state that you feel better, and the demo can now start.
6. Apply for a job as a fry cook.
5. Run screaming from the room.
4. Tell everyone the system is now up and running perfectly, then act perplexed when Bob is unable to get sound happening. Say things like, "It worked fine for me, I can't figure out why YOU are having trouble..."
3. Place calls to the tech support lines for each manufacturer involved. Since it is after business hours and they are closed, you can safely wait on the phone until everyone at the clinic gives up and leaves.
2. State that the world is too noisy as it is, and to help control noise pollution you are implementing a new silent demo policy.

And the #1 thing to try in this scenario:

Check the power switch on the mixer...