That's an interesting question that's never been posed to us before, and we haven't stayed at The Cosmopolitan and weren't aware of the chime, so we placed a call to the hotel front desk to see if they could identify the source. The very helpful lady we spoke with suggested the Guardian Angel Cathedral, which is located just off the Strip at 302 Cathedral Way and has a bell that chimes on the hour and half-hour, but that's located down by the Wynn, which seems too far to be audible at Cosmo, especially over the noise of the Strip. The Cathedral suggested The Shrine of the Most Holy Redeemer, which is located at 55 E. Reno Ave, next to the Tropicana and across from the Luxor. That's also a fair distance from The Cosmopolitan, but seems a more plausible source. If anyone out in QoDland has a better suggestion, please drop us a line.
As an interesting aside, both of these places of worship have been the subject of prior Questions of the Day, the former on 6/23/2011, which dealt with the construction of this architecturally striking cathedral, and the latter almost exactly six years previously, back on June 25, 2005, when we addressed a question regarding the souvenir chips that this church once sold in its gift shop. You can find both of these answers, along with a third QoD about clocks in Las Vegas (7/11/2008) in the Archives (open to LVA members only).
