I was at a concert yesterday and noticed that the lead guitar seemed quite quiet. My friend who was next to me also agreed with me, however other friends seemed to have no issues. My first thought would be somehow there was destructive interference, and stumbled across this question. I wonder, though, could destructive interference happen at a concert venue? One with tens or hundreds of speakers? Also if it was destructive interference, wouldn't it affect all of the instruments?
From Alex Feinman's answer he mentions dead spots, but I am not sure if that means that most of the sound would be cancelled out.
In a well-prepared concert hall, the space itself is constructed in a way to provide multiple bounces for the sound to arrive at every audience member. This is on purpose, to reduce the occurrence of destructive interference. Some halls have well-known "dead spots" where performers cannot be heard, or where audience members cannot hear; modern acoustic theory uses diffusers to avoid this.
This was at the Columbus Pavilion and we were standing just right of the set of left speakers (blue arrow)
Image credit Brock Bradley
For a better aerial view, here is the position in Google Maps.