This is correct. 99% of music is recorded in Stereo which is only front left and right speakers. The subwoofer will only play the bass.
If you want to listen to music out of all 5 speakers you must use stereo x2 mode which is crap!
If you want to test your speakers you will have to watch a movie with 5.1 audio or play a game that supports 5.1 sound.