iMovie is flexible.
The video and DVD standard is 48khz, so so if your camcorder has multiple audio settings, choose 48khz.
CDs and itunes tracks are in 44.1khz, and these can be used in iMovie projects.
I have never tried 96khz. I don't have any equipment that records at that rate.
As for bit depth, I am pretty sure it will accept both 16 and 24. There is no way to set this parameter in iMovie.
If you do live recording into Logic Pro, you can use 24 bit depth, and then mix that into a 48khz or 44.1khz recording in AIFf, AAC, Mp3, etc. and use that in iMovie. I am not sure about GarageBand.
If You want to make life simple and easy then do as I - convert all audio to
• .aiff - 16-bit 44.1kHz (Audio-CDs) or 48kHz (iTunes - I copy out as an Audio-CD .aiff (no .mp3) - never direct from iTunes into iMovie as DRM-Copy protection gives loads of problems even on Private Audio (readings etc.))
I never use anything else and there by omit's lot's of troubles e.g. DVD that goes silent etc.
Yours Bengt W