slow motion and audio

There will obviously be a difference in the length of the orig audio and the slo-mo video!
You can drag the same clip from the event viewer on top of the slo-mo'd clip in your proj and select the "audio only" option. You can do this multiple times to cover the entire length of the slo-mo' clip, if it makes sense ( e.g. Crowd cheering).
Remember to either reduce vol to 0 for the main clip, or use the ducking feature on the audio clip.

Be sure to use one of the pre-set "tick marks" for your slo-mo or fast-mo such as 25%. 50%, 200%, 400%, etc.

It you use the slider to create a custom percent like 57% or 83%, the Audio will be messed up.

I go back to the events , take an area surrounding the same clip that is long enough to fit the slow motion clip's length. Drag it up to the slow motion clip in the project and choose audio only when the dialog box appears. then adjust the length of this audio clip to fit. Remember to set the slow motion clips audio to zero in the audio adjustment menu.

Covering the slow motion audio used to be a real pain in iMovie6 and is now one of the best features of iMovie9.

I make mostly sports videos so getting background sound from anywhere in the events is easy.

you could also use stock clips of ambient sound from the sound effects area