I think you mean that the middle pin is grounded and each of the others goes to the negative of a component to turn it on. In the middle position neither are turned on and of course both cannot be turned on at once.
If this is the case, you can use something vaguely similar to MarkT's circuit except that you must use a logic-level FET, not a transistor and the gate is connected not to the centre switch contact, but to the anodes of two diodes, the cathodes going to each of the outer switch terminals.
You will also need a 10k resistor from gate to the supply terminal of whichever is the higher voltage.
What is peculiar is that you would ground the negative of an audio device to turn it on. You might care to explain how this works. 