On one of my trips to Luxor, at Habu, I accidentally discovered something similar, Hmmmmm...
Chris Dunn did some experiments with sound in the Great Pyramid in the Kings chamber, interestingly enough, I was chatting to someone, who back in 1978 conducted similar experiments with the so called sarcophagus and got a different result to Chris...
Headphones area must..

Use the topic title to search for more if you are interested...
Granite, very dense, excellent acoustics, and as far as Monoliths are concerned, waaay too much effort for the purposes of "decoration"...