I know why custom soundtrack in FIFA 11 doesn't play next song!
Out of boredom, knowing that I have powerful foobar2000 with vgmstream plug-in, I started to listen to various FIFA soundtracks. The point is, with FIFA 11 music, .sns file is not enough. foobar doesn't recognize this file.
It needed additional file hidden in audiodata/musicdata/music_snr.big This big file contains .snr files. They're very small (8 bytes) and they are named by songid (x.snr), so it is probably some type of indicator or timer for specific file, where to stop.
So if song is longer or shorter than expected, the code doesn't understand what to do, so it does nothing.
I theoritize if we were be able to prepare such file to specific song, it would play next one after finish of the latter. I upload a sample file to analyze.
Btw. 2010 World Cup game uses this as well and they're compatible with each other, so my mod can have (besides two additional songs) a fully working soundtrack.