This is most likely due to one or more channels being soloed. When an audio channel is soloed, the remaining channels that aren’t soloed are automatically muted so that you can hear only the soloed audio channel(s). Press the SOLO(PRE FADER) button for each channel (1-12) so that the each button is pressed outward (i.e. SOLO will be off)--in order to hear audio once again. The SOLO(PRE FADER) buttons are located directly above each channel’s LEVEL knob.