It's not a gray area at all
The copying of copyrighted music for anything other than personal use is a violation of the copyright.
Which includes distribution with a slideshow or performance to an audience.
To be legal, the client would have to negotiate a license with the copyright holder - you would contact the performing rights organization (or in this case, the mechanical rights organization) that licenses use on behalf of the music publisher/composer. Technically, what you are looking for is a syncronization license, because the music is being used together with visual images in a presentation/movie/television show etc.
CMRRA is the clearinghouse for most publishers in Canada for mechanical rights and synch rights. What is CMRRA?
Also, beware of using a copyrighted work in a context that implies that the artist is giving an endorsement of the client's product/service/position