> I'm honestly not convinced I read a less "overengineered" solution so far...
Prerecorded music on an MP3 player sounds like the one simpler solution, though it wouldn't put the elevators in sync with each other or let you control it remotely.
Yes, but if you want to do even as much as not play the music in the middle of the night, now your MP3 player needs a clock. That clock may drift, or it may drop out completely when the power goes out, unless you synchronize that clock over the network. But if you have network on your MP3 player, you may as well let it subscribe to the multicast stream instead of reading the stream from internal storage. If you do not have network, and you want to change the schedule of when your music plays, have fun to send out some maintenance crew to every single one of those devices. And don't forget any...
In the U.S. at least, most jurisdictions will require the music to be shut off in an emergency -- namely, if the fire alarm goes off. Audio sources commonly are required to have a relay/GPIO input that triggers the shutoff, a network command is not good enough (network switches typically not being life safety rated).
In this hotel scenario, you have one audio source that can be located in a location that can be conveniently tied into the fire alarm relay panel. Stop the audio there and you've stopped it everywhere.
Prerecorded music on an MP3 player sounds like the one simpler solution, though it wouldn't put the elevators in sync with each other or let you control it remotely.