Recording a live voice in the room then playing it back on headphones will often reveal problems which the brain filters out when hearing it live. Listen to some awful-sounding system recordings on YouTube - in person they probably sound quite good, so the brain is working hard to process the anomalies, something it will not need to do so much in a properly treated room, making playback less fatiguing.
As MrKlinky said, listening to a live recorded voice over headphones will be highly revealing of the room's issues - especially if the microphone is 8 or more feet back from the vocalist. A funny story I heard involved a recording technique where a singer is close mic'd. and then a second mic. off in the corner of the room is also used. The two signals can be mixed later to add room sound if desired. The mic. near the corner will have a lot of room sound in it. In this case they had a very well treated tracking room that used combination bass absorbers, treble diffusers that had a curved surface and were placed all around the room - TubeTraps. The recording engineers were having trouble telling the difference between the sound of the close mic. and the mic. all the way off in the corner of the room! That's how neutral and effective the room treatment was. I should disclose that I sell these acoustic products.