FIFAMOD tool is slightly buggy, but it still works well and does help/do the trick. I also did my own "research" by opening the .sbr file in hex editing software and have understood how it works and how to manually change the soundtrack. The .sbs file is simply all audio tracks (in ealayer3 format) added together in one big file. You can add more song entries into the game, however I don't know what the maximum amount is and/or what "songids" should be used, since I have seen that (for some reason, maybe an important one) EA themselves skip certain "songids" (maybe because they had another song ready for that id but had to cut it out or something or because the memory being read from that id is used for something else) and by trying myself to add new entries to the soundtrack in the database (on FIFA 19 PS3), certain issues started to appear like, wrong elements being loaded and wrong strings appearing (as if, like I said above, the "memory" used for the new songid was used for something else and therefore broke something).
However, I found another "trick" to have double the amount of songs already in game. For example, on FIFA 19 (PS3), there are 43 songs. I was able to give assign 2 songs to ever song entry. So for example if you open EATrax and select a song from the list, 2 songs will be played (randomly) from that one entry. If you want, I can explain you how to in a PM.
As for the gui, one thing you should be able to add are the "stadiumintrovideos" (as it's called on the World Cup 2014 game), which shows the stadium the match is played in when you start a match in a video.