That could have turned out so much worse for all involved, but I suspect those involved had different perceptions of the risks than we do now. Though there was still some risk of dying, this must have seemed like a cakewalk compared to jumping out of a WWI trench!
Some Greek chanters got permission to go into the Hagia Sophia and record a TV special (for Greek TV) of chant music several years ago. While not a service, it is the music that would have been used at a service. (Either that, or this is another case of convolution reverb and some clever video editing. I'm not sure. But it sounds pretty cool.)