For the sake of limited space I'd like to connect two sound boards to one speaker. The boards would never be powered on at the same time, but I'm concerned that the voltage of one would travel through the speaker back to the other and damage the other board. Would diodes limit the flow so that wouldn't happen?
Terry
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personally, I think you're asking for trouble! but what you could do is us a DPDT switch and just throw the switch for whatever board you want to use! why not just use one board or add a second speaker one for each board if you have the room in the engine!
Alan
If the boards are never powered at the same time, how about an automatic approach?
Connect a DPDT relay to the power source of one of the boards. Connect the speaker to the two common leads of the relay contacts. Connect the speaker output of the board that controls the relay to the N/O contacts of the relay. Connect the speaker output of the board that doesn't control the relay to the N/C contacts of the relay.
This will totally isolate the speaker from the non-operating board, and allow you to have either one driving the relay.
Thanks guys. It looks like it will come down to size. Either the 2nd speaker or the relay, whichever fits better.
Terry
Well, I can't imagine a relay being bigger than a speaker that would make enough sound to hear it! 
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