Check the following:
- Is the power switched on?
- Is the LINE switch on the front panel switched out?
- Are the INPUT and OUTPUT LEVELS set correctly?
- For microphones that require phantom power (ie condenser microphones), is the +48V switch engaged?
- Is something connected to the Instrument input? (This overrides the mic input.)
- Are headphones connected? (These mute the monitor outputs - see the article regarding latency-free monitoring)
- Is there something connected to insert return?
- Is the Optical Expander activated?