After sorting out the Yamaha S80 connections, there was still no audio getting through the 828.
If the connections to the 828 are not working, you can find the firewire cable connecting it to the iMac, remove it and re-insert it. In this case, that did not solve the problem.
I checked the audio devices tab in the Audio/Midi setup (a utility with a squareish music keyboard icon). MOTU 828 was selected. So, what to do?
Restart. (find the blue apple icon in the upper left corner of the screen, pull down the menu, and select Restart).