Sound is propagated by waves. Waves can interfere.

Suppose there are two tenors standing next to each other and each singing a continuous middle-C.

1. Will it be the case that some people in the audience cannot hear them because of interference?
2. Would it make a difference if they were two sopranos or two basses and singing a number of octaves higher or lower?
3. How does this generalize to an array of n singers?
4. Given a whole choir, to what extent are their voices less than simply additive because of this? Is it possible that, for some unfortunate member of the audience, the choir appears to be completely silent--if only for a moment?