Hey Forgiven,
If I understand correctly, you expected the music to fade-in, like it does in the YouTube video.
The music in the pack doesn't contain a fade-in, so that it can be looped properly when desired.
If you would like the music to fade-in with volume instead, you can do this in the dedicated game engine that you work with or use free tools like Audacity to do so.
I hope that helps!