Often, in stereo music the singer's voice is equally on both channels, but the stereo position of other sounds varies. If you connect a speaker between the left and right channels, you won't hear the singer. Normally, you would use two speakers, one connected between left channel and ground and the other between right channel and ground. If you use two speakers meant to work like that, but the ground connection is broken, you in effect have two speakers in series between the left and right channel and you also won't hear the singer.