Just to be clear, so far I have been compiling the background music in my slideshows for clients. I am just stating the underlying intent that by requesting the clients to provide their own music to me, my job is to sync the slideshow to the music that they appoint.
But now from what I gather from vince and TheQuestion responses, it is definitely a great idea to provide 2 slideshow versions to clients: one without music, and one with music. Therefore, my liability is only to provide a music-syncing service on top of my visual slideshow creation. I can also argue that I am charging the clients for the slideshow without music, while the version with music is done as a personal favour.
In similar context, I have clients requesting me to edit/remix copyright music to fit a certain length. My approach is that I am providing a service to mix music but I am not responsible for the copyright that the client has to bear when playing the music in public places. I also do not SELL the copyright music - meaning the client must provide the music for me to edit.