@eman
As I stated not all municipalities have noise ordinances, so it doesn’t matter what level you record with an app.
#2 The guests have no standing, they did not hire the band. If they don’t like the noise level they are free to leave.
The last 2 weddings we hosted were in the CTL Compound out of doors. The music is not held in by walls and ceilings and doesn’t seem as loud as in a confined place. The same volume settings in an amp have different effects on the audience depending on venue. The worst experiences I’ve ever had are at the basement wedding halls in Brooklyn, the assorted Ateres whatevers that have low ceilings and no sound deadening material on walls, floors, etc.