Might be a corrupt preference causing the issues. Try deleting preferences. It is a safe procedure that won't cause loss of data or any disruption to your projects. To delete preferences open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects. Now see if iMovie works properly.
You could also try booting up in Safe Mode (see Finder menu for instructions) and, after everything loads, immediately shut down and reboot in normal mode. (Don't try to open iMovie in Safe Mode, because it won't.) Then see if iMovie works properly.
Also, update your iMovie app and operating system to most current. I believe that Big Sur 11.5 is the most current.
EDIT: If the above doesn't cure the problem, would you describe more specifically what steps you are taking, and what exactly happens, when you try to edit the audio and adjust the duration of transitions. Is there any specific procedure you are doing when the crashing occurs? Any error message?
-- Rich