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 they are all pressed outward, in order to hear audio once again. The SOLO(PRE FADER) buttons are located directly above each channel's LEVEL knob.
If the MUTE MAIN MIX button is flashing, check each channel's SOLO(PRE FADER) button carefully and make sure every channel has the button pressing outward.