newbie dumbness, I think

Are you filming in slo-mo? The casio does not record audio in slo-mo to my knowledge. 

Wish I could help you out more. 
When I want sound and slow motion, I film in regular speed and then change the playback setting. The big downside to this is losing the details captured in the higher frames per second. I wish I knew a way to have both because the sounds add so much to a video. 

"The big downside to this is losing the details captured in the higher frames per second. I wish I knew a way to have both because the sounds add so much to a video."

Think about about what your asking for a second... How the sound would be heard playing it back in slow motion. Your voice or any other sound would be very distorted. 

What I think you really want is to add an audio track. Surely there are free/inexpensive programs available to do this as a "post production" step.