Hi Raul,
Thanks for reaching out in Apple Support Communities. I understand that you're attempting to set start times for certain songs in your Music library that you've imported from other sources, but the changes you're making aren't saving. I know it's important to be able to customize your library; I'd like to help. When you select "Get Info" for a song to change the track start time under Options, is "Remember playback position" checked? If not, check the box and test to see if your preferences are saved.
If the option is already checked, try these steps next:
- Uncheck the "Remember playback position" option for the song(s) affected.
- Quit the Music app on your Mac: Quit apps on Mac
- Restart your Mac
- Open Music while holding down the Shift key on your keyboard; when the icon bounces in the dock you can release the Shift key.
- Edit a song start time, check the "Remember playback position" box and test again.
If the behavior continues, try testing in a new user account and let me know what you find: How to test an issue in another user account on your Mac
Best Regards.