Hey there, Nuno!
Thanks for visiting the Apple Support Communities. I see from your post that your 2019 MacBook Pro is exhibiting crackling sounds from the left speaker, and the audio isn't as good as you expect it to be. I'm happy to help!
It looks like you've already tried resetting the SMC and NVRAM on your computer, as well as using Apple Diagnostics. These are great steps to take when you're having an issue with audio/speakers. Since the problem continues, I'd suggest going through the following steps to help isolate the issue to hardware or software:
- Restart the computer in safe mode, and see if the issue continues there: How to use safe mode on your Mac Safe mode runs some checks on your system as it boots, and makes repairs if it can. If the issue continues after that,
- See if you can replicate it in a new, test user account: How to test an issue in another user account on your Mac - Apple Support. This will let us know if the issue is related to your specific user settings or content. If it does occur in a new user,
- Boot into macOS Recovery: About macOS Recovery - Apple Support, and select the "Get help online" from the Utilities pane. This will open a Safari window and you should be able to navigate to a site that has audio from there. See if the issue occurs here. macOS Recovery runs separately from the actual macOS software, so if the issue is related to said software it will not be present in macOS Recovery.
If the issue does not occur in macOS Recovery, try reinstalling the macOS software: How to reinstall macOS from macOS Recovery - Apple Support.
If the issue does occur in macOS Recovery, and/or reinstalling macOS does not solve the issue, please get in touch with Apple Support directly. You can do that here: Contact Apple Support.
Best!
