So, a quick tutorial on how to change your Steam CT music
You need:
Sounds of IvaliceCT explore (archived latest, not sure what happened to RiverNyx, but his website doesn't appear online anymore, so thank you!)
Wavosaur- Locate resources.bin with CTExplorer;
- Extract the BGM, found under Sound directory;
- Decide which ones you want to replace and find the replacement;
- Convert your replacement to WAV, open with wavosaur and create a loop point;
- Open Sounds of Ivalice, open the original .sab file you want to change, select it, right-click on the audio and select "replace", it will point towards your WAV, replace the file;
- Use CTExplore to replace the old .sab file with the new one you created;
- Repack and close, and it should work.
Be mindful that this is only a replacement, it is not possible, as far as I know, to add entirely new music and have it recognised by the game.
EDIT: I also recommend looking at the music file you want to replace in an audio editor, if you have it, and normalize the volume so that the replacement has more or less the same loudness of the original file.