Figure 1 depicts the following scenario...
Tape 1 (which is the Mac Input) is set to be monitored on the Tape 2 monitor output. This means that any audio sent from the Mac to the Amp is being passed to the Tape 2 monitor output instead of the speakers mounted to the wall, therefore, no sound will come through the speakers.
The arrow is pointing to a part of the display that states Tape 1 is being Monitored on Tape 2.
Figure 2 shows you what the display should look like when everything is set properly on the Amp. If you find that Tape 1 is being monitored on Tape 2, as is shown in Figure 1, tap the Tape 2 Monitor button to turn this option off.
The arrow on Figure 2 points to the Tape 2 Monitor button.
If you are still not getting audio, verify that the Mac sound is not muted. F10 will toggle between Mute and Un-Mute. F11 turns the speaker volume down and F12 will turn the volume up.
Another item to check is to make sure that the Sound Preferences are set to the correct Output.
- Go to System Preferences -> Sound. Once you are in Sound Preferences, click the Output tab. Make sure that Internal Speaker is selected.