Sometimes mixes fall apart in mono, for a variety of reasons. Why is this important? Who listens in mono anyway? We all do, all the time. If you are not sitting right in front of your speakers, you are hearing some form of mono. If are are listening from way off to the side, or from another room, it’s now mono. Unless you are right in front of the jukebox, you are listening in mono.
The majority of TVs are mono. If you are listening to a boombox from more then a dozen feet away, it might as well be mono. And don’t forget that most sound systems in clubs are run in mono. And lots of people wake up to clock radios. And so on…
That said, here are the pitfalls to be careful of. Any instruments that have been recorded with more than one mic, such as drum overheads, acoustic piano or guitar, choirs, etc., AND panned wide apart, need to be checked in mono, as there may be phase cancellations that dramatically reduced their perceived level in mono. I have seen cymbals and pianos utterly DISAPPEAR in mono. This can lead to severe inconsistencies when your mix is played in different places.
There are several ways to watch out for this. The best place is when you are doing the initial recording. Some of the more expensive consoles have phase meters. If you are using Pro Tools, etc., there are plugins you can download which will do the same thing. Put it on the master fader, and solo your stereo track. The phase meter should swing to the right when there is sound. If it stays in the middle, or swings to the left, try moving the mics and checking again. You can also check with your ears by switching to mono for a moment, but the metering is much easier and more reliable. For “after-the-fact” phase canceling issues, simply narrowing the panning and raising the level a bit often fixes it.