Hello dannn1984,
Thank you for using Apple Support Communities. If you've used this feature in the past without any issues, you might need to adjust the pitch of the audio. This resource explains more: Adjust video speed in iMovie on iPhone - Apple Support
By default, iMovie preserves the pitch of audio clips that are sped up or slowed down. To change this, tap the Project Settings button , then tap to turn on “Speed changes pitch.” This allows a recorded voice to be higher when sped up or lower when slowed down.
If making adjustments to the pitch of the audio doesn't help, you might want to test a different video clip to further isolate this issue. Another step you can try is to uninstall and reinstall iMove on your device. This can also help address unexpected behaviors you might encounter while using the app itself.
1.) Delete app
2.) Redownload app
We hope these steps are helpful. Thank you for using Apple Support Communities. Cheers.