Hi Kathryn,
I've only seen an option for sound on, or sound off too... Also, as there is no option of using an external Mic you are pretty much stuck with what you have regarding video sound.
You could record with sound off, and use a separate device for the sound recording then dub the sound track over the video in post-process, but keeping them in sync would be difficult.
Alternatively, once you have recorded your movie, I would think many video editors would allow you to simply reduce the volume of the track.
I use the video function of my 3100 as a "Nice to have", but if you are serious about taking lots of high quality video, then a dedicated video camera is the best way to go.
Steve