There have been a few of these copyright threads relating to music in slideshows and montages; I've gotten the general sense that many photographers are reluctant to compensate the composers and performers of copyrighted music for using their music in montages / slideshows, which seems at odds when the same photographers want their image copyright protected and compensated for when the photographs they take are being used (I think more than enough has been debated about commissioning parties / work-for-hire). Surely, if photographers want their image copyrights to be fairly compensated, then they should show the same respect and financial consideration to content creators who operate in a different medium.
Hotels have licenses for public performance of music on their premises, and by public performance, that means a pianist playing covers right through to original recordings being played over a PA system. What that license does NOT cover is synchronization rights: when you put make a montage of images and synchronize music to those images, you need permission from the copyright owners that is not covered by the hotel's license. You could play a piece of music using a manual synchronization (you press play for the montage coming off your laptop and then press play on a separate CD player / CD playback app on your laptop) and that single instance of copyright music playback against your montage / slideshow would, I'm fairly sure, be covered under the hotel's license, but only there and then in the ballroom. The synchronization rights still apply when slideshows / montages are passed to the wedding couple &/or uploaded on a website, but by this point, you also have other issues such as mechanical copyright (making physical duplicates of music) in addition to the performance rights.
The only solution is to pay for using the music, and COMPASS rightfully acts as the agent for these performance rights (they are affiliated with ASCAP & BMI (US), PRS (UK), CASH (HK), etc. and you will not find any copyright music wherein the composer and their publisher hasn't by automatic prior assignment assigned the performing right society to which they are a member, for that performing right society to act on their behalf globally through these affiliations).
So who pays for this? Ultimately it will be the commissioning client (i.e. wedding couple); there really isn't a way around this if you are to do right by your fellow creative brothers in the music field, but you do have an option in terms of what you mark up on organizing the license(s) for the music you want to use. Sure, no-one wants to be forced to raise their price to when pricing can be a factor in deciding for couples to decide on who to shoot their wedding pictures, but when you look at things from the position of the copyright owners who are losing income when they are not compensated, there really isn't any other option if you want to use copyright music in montages / slideshows for weddings.