Don't bother trying to fine-tune. Use this time to make sure all is as it should be. Experienced bands usually play a polka, schottische or other energetic couple dance to help with this before the caller begins, so you have fewer variables.

Next check that all mics are working. At a minimum, tap each mic (don't blow into them - it's ugly sounding and not the best thing you can do for the mic).

Continually remind yourself to check the balance of the band and caller. If you're primarily a musician, remind yourself to check regularly how distinct the caller is in the back of the room, especially if they do a lot of squares. Could you dance to it? If you're primarily a dancer, make sure you don't have the caller booming out and the band meek - don't knock the stuffing out of the music.

Are all of the instruments audible at the back of the hall? If not, a mic or cable may not be working, or turned up.

Hums or buzzes? Look around for dimmer switches on lights, and either turn them off or all the way up. Flourescent lights on the same circuit as the sound gear can also cause problems. Make sure you're not using an unshielded cable where you need a shielded one. Turn channels down one at a time, briefly, to find a bad cable or other problem.

Monitors
Are the monitors working, and can the band hear each other adequately? Don't wait for the band to complain - get to them as soon as possible, listen from behind a player to make sure the monitors are putting out enough for them to hear, and ask them at an opportune moment how the monitors are. A lot of problems can be solved by adjusting monitor placement, so try this first.

Look before Twiddling
If any EQ knob is turned more than 90° off center, or a volume knob is turned as far as it will go in either direction - watch out! Settings like this usually mean that something else is wrong, and you just haven't spotted it yet. Look before you twiddle! Trace and check all connections again.

Emergency Response

Feedback
For feedback, reduce the monitor level fast, and be ready to immediately reduce the main speaker level if that doesn't work. Then form a theory as to what actually was causing the problem, address that, and bring the levels back up. Hint: inexperienced players put their hands over the mic if there's any feedback. This immediately generates additional feedback, so make sure you stop them.

Most feedback is a function of the shape of the hall and stage, and the placement of the people on stage and the speakers and monitors, rather than a mic aiming at a speaker. It usually is a problem at low frequencies, since objects are opaque to highs (where most of the sound information is) but allow bass to pass by.

Missing Sound
For missing sound, check the obvious again, before turning things up to unrealistic levels. Are the plugs at the mixer and speakers inserted all the way? If a specific mic isn't working, try re-plugging it to be extra sure. Cables and mics can spontaneously stop working. Before assuming this, though, check the battery or switch on the mic, if there is one. Check the mono/monitor switch if using an Entertainer system.

Missing instrument
If an instrument isn't coming out, and you've checked the obvious, be ready at the end of the first dance to replace the mic and/or cable to that instrument. You can try just unplugging the mic and re-inserting the cable into it, but don't waste any more time than that trying to fix it - just replace it with one of your spares (you do have one, don't you?).

You may run into sound people who think this is also a good idea for the hall mix with acoustic music - it's not. You want to get the sound as good as possible, not just as loud as possible, and this technique responds only to the resonant frequencies of the hall, not to what we want to hear from the music. But sometimes people resort to it to deal with...