This facility appears very restricted - it only allows one file to be used and only in mp3 or mp4 formats. Most recorded music is in separate tracks and most slideshows will be more than 10 mins duration so unless one has a single track long enough, the pictures won't fit the music. I tried stitching together several tracks into a single file but LR didn't regard that as a valid file for some reason. It would be much better if a whole folder of music tracks could be selected and each one then played in sequence or randomly. Also for other formats than mp3 to be allowed.
The Slideshow module in Lr is nearly a laughable offering compared to other options ... it has languished in obscurity since almost Lr 1 ... I have a 99 cent app on my iPad that can blow it out of the water by comparison ... Adobe should either drop the module completely or open up the SDK and let someone else try to offer a viable and reasonable solution ... because what they have offered thus far is a waste of effort and good hard drive space.