QuickTime Player crashes after updating to macOS 15.7.1

Try this:

QuickTime Player > Services Settings... > Restore Defaults.

Or make a dummy user and see if Quicktime Player works in that account.

If that works, try to reset QuickTime preferences in your account: Quit Quicktime Player. Choose Finder > Go > Go to Folder... [Shift-Command-G], paste the following line, and press the Return key:

~/Library/Containers/com.apple.QuickTimePlayerX/Data/Library/Preferences/

Then try if moving QuickTime preference file from that folder to the Desktop folder helps helps (just in case before deleting it):

com.apple.QuickTimePlayerX.plist

This might also be worth trying:

Settings > Desktop and Dock > Click wallpaper to reveal desktop: "Only in stage manager"

Settings > Desktop and Dock > Stage Manager: OFF

I'm unsure what is causing your issue, but you can try and Set up users, guests, and groups on Mac and see if the issue persist in the new account. that will tell us wether it is an issue with your user account, or machine wide.

if no resolve, are you using any apps that claim to "clean up" or "speed up" or otherwise "optimize" your Mac? and / or any antivirus apps? the only thing those apps are good for is causing major issues within macOS while providing ZERO protection or benefit. you would do well to uninstall them.

if you have no such apps, i'm thinking you should download and run the free version of EtreCheck so we can see if you have some software installed that is causing your issue. make sure you give "full disk access" to etrecheck. Learn how to use it by reading Using EtreCheck. if you need help interpreting the report, you can see how to post the report here by reading How to use the Add Text Feature When Posting Large Amounts of Text, i.e. an Etrecheck Report. and it automatically obscures sensitive things (like serial numbers) so you don't have to worry about sharing the report here.

Something must have gone very wrong doing that update. You might call Applecare 800-275-2273 for their personalized assistance. I've had good experiences with Apple support who may screen share to see the whole picture. Walking myself thru what you are doing, this is my experience:

Editing mp3 audio files with quicktime converts them to .mov files, then you can export as an audio file, to m4a.

Editing m4a audio files saves them as m4a files. Also, m4a file can be a video/audio file.

Creation of an audio file is saved as m4a.

I frequently use this function as I need to see the sound wave.

A much more detailed sound wave, if such would be useful to you, is available in specialized audio processing apps, e.g. Audacity.