@VanallesennogwatCustom music shouldn’t be an issue. Make sure the song files are in the MP3 format otherwise the game can’t read them. Same for song files bought on iTunes.
***Edit***
Have you deleted your Accessory.cache and Group.cache files before you start playing? These could also be causing the loading screens to freeze.
I found this thread on the Official Forums that mentions some other solutions to try to see if one of these may be causing your loading screen freezes. https://forums.thesims.com/en_us/discussion/886574/the-sims-2-ultimate-collection-stuck-at-loading-screen
