Hello there. I have a few things to ask about.
First off, how is your JSON file named? It should have ".sound.json" at the end of the filename. Your soundtrack JSON file should be called something like "MySoundtrack.sound.json" and not "MySoundtrack.json".
Second, where is your JSON file located? It should be put in the directory "C:\Users\%username%\AppData\Local\THUG Pro\User\".
Third, what does your THUG Pro User folder look like? Something in that folder should explain why you have another soundtrack called "ThugPro 2 Soundtrack". Maybe there's a DAT file you forgot to delete? Because what THUGPro does is that it uses JSON files to generate DAT files, and then it reads the data from said DAT files. You may have a leftover DAT file that has the data for this "ThugPro 2 Soundtrack".