In article , Fill X
wrote:

I often notice headphones make people tune their instruments sharp and the mix
is key for a singer certainly, but different singers usually find what works
for them and it can vary a lot. I do more often hear women sing sharp and men
flat than the other way around.

Interesting. Perceived intervals expand as you listen louder and shrink
as you turn the volume down.

I had a chance once to experiment on a bunch of complete novices in a
class I taught with a vocal coach. Without exception, they all sang way
better in tune when none of their vocal was being fed to the phones.
Too bad we can't talk most folks into recording that way.