Voice-over narration for videos

I've had pretty much this exact same situation before.

In my experience, it was a lot better to cut each video/relevant audio together in some other software (in my case I used Camtasia, not iMovie), publish that content as one video, and then import that  file containing both audio and video into storyline.

I found the end content worked a lot better when it was done this way, because the audio and video was never at risk of getting out of sync due to any of the files lagging within the storyline file. Whereas if you just have one video and it lags, the audio and visual content still stays in sync.