I have Creative Labs Soundblaster Live! (value) soundcard
$ lspci | grep audio
02:07.0 Multimedia audio controller: Creative Labs SB Live! EMU10k1 (rev 06)
It has two analog line outs: green and black one. I can get sound of only the green one in Gnome session. In Xfce4 session I get sound from both; the black one is controlled by Wave Surround mixer setting. I suppose Gnome is using pulseaudio, but Xfce4 doesn't. I wonder how can you enable the black output (together with the green one) in Gnome/Pulseaudio?