The Ghost of FM said:On the right hand side of the board, there is a dedicated tape sub-mix system with 8 pots and tape switches just above the 1-4 buss master faders.
If the tape buttons are depressed and on, it will channel the tape returns away from the first 8 channels of the mixer and instead route the sound to the stereo buss if they are switched on active.
This sub-mixer is used for creating auxiliary mixes when you need the main mixer strips for creating complex tracking mixes like Mic'ing a drum kit with several Mic's and still hearing what is on the tape tracks at the same time.
You also have to double check that all the main mixer channels are turned on, that solo buttons are off and that each channel is switched to the desired input and routed properly though the 1-4 and stereo buss buttons on each channel.
The M300 series of boards are a very complex and full featured mixer and if you are not familiar with it, there are a few sand-traps that are not intuitive to get out of without the manual or previous experience.
Let me know if that solved the problem for you!
Cheers!