audio clip is getting shortened to fit video length

Hi

I have imovie 10.1 My project currently has 6 minutes of video. I want to add a 10 minute audio clip to it, and then edit the audio down.

When I add the 10 minute audio to my project, imovie shortens the audio clip so that it ends when the video ends.

I know I've made projects before where the audio could be any length in relation to the video, and this shortening did not happen. I'm not sure what is different now, or why.

Does anyone know why this happens and how to stop it?