Quicktime edit "add clip to end" no audio with added clip

Hi 71nothing,

We appreciate you reaching out to us.

We’d like to gather additional details that may help us be in a better position to assist you.

Are you using the steps below to make the edit?

1. "In the QuickTime Player app  on your Mac, open the movie that contains the clip you want to add. 
2. Choose View > Show Clips.
3. Select the clip, then choose Edit > Copy. The selected clip is outlined in yellow.
4. Open the movie that you want to add the clip to, then choose View > Show Clips.
5. Choose Edit > Paste. The clip appears at the beginning of the movie. Drag the clip to rearrange it.
6. When you’re finished, click Done."

You can use the article below as a reference.

Add a movie or clip in QuickTime Player on Mac - Apple Support

If yes, please test if the issue persists on a newly created user account. 

1. "On your Mac, choose Apple menu > System Settings, then click Users & Groups  in the sidebar. (You may need to scroll down.)
2. Click the Add Account button below the list of users on the right (you may be asked to enter your password.)
3. Click the New Account pop-up menu, then choose a type of user.
4. Enter a full name for the new user. An account name is generated automatically. To use a different account name, enter it now—you can’t change it later.
5. Enter a password for the user, then enter it again to verify. Enter a password hint to help the user remember their password.
6. Click Create User."

You can use the article below as a reference.

Add a user or group on Mac

Take care!

Hello 71nothing,

Thanks for the update. If the previous steps didn't help, we suggest you contact Apple Support. They'll be able to look into this with you further.

Have a great day.