The sound of the Hagia Sophia, more than 500 years ago
The next step (that will probably never happen) would be to refurnish the Hagia Sophia, as it would have been before it was plundered in the fourth crusade, and seat an audience in it, as if for a mass.  Then pop another balloon.  All those extra furnishings and bodies would have had a significant impact on how the space sounded.  The plastered over mosaics might also create a noticeable effect that would need to be compensated for.  Even with the impressive work of Abel and Pentcheva we still have something that would likely sound empty and strangely hollow to people familiar with the Hagia Sophia when it was serving as a church.