If you save your audio commentary in one of the supported audio formats (such as mp3) you can add that audio track. There's no requirement that the audio be music.
You can adjust the length of the slideshow to fit the audio track with the "Fit to Music" button in the Payback panel on the right hand side of the Slideshow module.
For a voice commentary with a slideshow I have recorded my voice using Audacity as I am viewing the slideshow. This method fits the audio to the slideshow. Then add the audio to the slideshow in LR.
Thanks for your help. I finally selected iMovie and am satisfied with the choice. Each photo is treated as a movie clip that one strings together and adds audio. I am using iMovie for the first time, so it is slow going.