So I spent about three hours trying to find the cause of this problem, and finally managed to track it down. It is the module by LittleMsSam called “LittleMsSam_Revy_SynchronizedStereos” from the main script mod called "LittleMsSam_PersonalObjects". And yes, the mods are updated to her latest.
This mod/module is responsible for the S-pop radio station no longer working properly. I tested every single modification I have in the game, and even though I removed them all for testing and left only the few MP3 files, the radio station remained silent. So I created a new save file and tested only with these specific MP3 files, and the station works again as before, WITH MY MP3 files in the aforementioned folder. When I add just this mod back, the station goes silent again.
Somehow, this SynchronizedStereos mod must have corrupted my save file, which is why it didn't work even when I removed ALL my mods. When I add this mod back to my new save file and then remove it again, the station remains silent. If I create another new save file without this mod, the station works as intended again and plays my custom music.
So the culprit for all of this is the LittleMsSam_Revy_SynchronizedStereos mod. I deleted it and created a new save file since it corrupted my current. Problem solved.
If anybody is wondering why their custom music wont play anymore in this specific S-pop station, (a lot of ppl using custom music) this mod is the reason why.