There is no need to apologize for the amount of threads you've created, Raikaze. If you need help, you are always free to ask the community and we will try to offer any solutions or suggestions on how to fix the issue. <3
To install the music, copy and paste those MP3 tracks into your Feralheart > media > music folder. I can assume the CFG file is titled music.cfg? If so, place that into your main Feralheart folder, replacing the original music.cfg file when the option pops up. I suggest you make a backup of the default Feralheart tracks as well in case you're wanting to switch back.
As for the new interface, the download should have come with textures arr? Place those files into your Feralheart > media > textures folder and click replace. As I've said before, be sure to make a backup of the default files in case you are wanting to revert back to the original interface. Hope this helps you out a bit.