install the comentary and change the settings in the locale.ini
first open the notepad and save it as locale.ini in fifa 08 main folder CProgram files\EA Sports\FIFA 08\... Then open the locale.ini created and put this:
Backup your original audio.big, (eg. rename it into audio.big.bak)
and copy the audio.big which came with the spanish commentary,
in your \FIFA 08\data\audio\...
and try again, it should now work, with the settings dave5000 told.