Set the Main Out knob around 2:00 and turn up the speakers of course. Try setting this knob around the 1:00 O'Clock area for starters. PB = Playback as in whatever your DAW (or Windows Media Player, Youtube, etc.) is playing back. IN = whatever you have plugged into INPUT 1, 2, 3, or 4. The 'MIX' knob is for blending between IN and PB. For this reason I suggest you simply connect your speakers to the Main Out jacks only. If I'm correct you will only be hearing the computer's playback which will include latency of whatever you are recording. If you use only the Playback Outputs I don't think you will be hearing the front inputs directly to the speakers as in 'direct monitoring'. If you use only the Main Out jacks you will get sound from the front Inputs and also the Playback from your computer. You should have some type of powered speakers plugged in to the rear Main Out jacks or Playback Outputs. You haven't mentioned what you are using for monitor speakers. Playback does not sound - even when I do see the meter moving.'
