If you have an M-Audio Audiophile 24/96 PCI sound card or anything else using the "Envy24" chip, be aware: at present neither the GNOME3 "sound" control panel applet nor the pulseaudio tool "pavucontrol" expose enough of the card's controls to play any audio. You need to have alsa-utils or alsa-tools-gui installed and use envy24control to put the relevant sliders in the right places.