[boomlinde]

> Assuming headphones and instrument/vocal mics that are within a few inches

It is relevant, and monitoring via headphones or a floor speaker is a mitigation.

It's also a problem for large orchestras. If everyone else just plays along with what they hear from the people near them, the timing is going to be inconsistent across the orchestra pit even if you have some obvious audible reference like loud percussion. The mitigation is to orient yourself as much as possible around the visual cues from a conductor.

[mikepurvis]

That's a good point, and I'd imagine instruments like a church organ would suffer from this too, though I know in that case there are a bunch of other sources of delay, like the time it takes for the pipes to fill with air.