Passive audio phase shifter (180°) - can it be done?
Although it's not so clear to me what you want I can point you to something that might help. 
Old telephones used a passive device, a transformer, to separate the incoming signal from the outgoing signal (so that your own voice is suppressed while you're speaking on your phone).
This is a schematic of a T65 made by Ericsson. Almost everyone in the Netherlands had such a phone in the 1970's ;-)



What is interesting is that the signal from the Microphone is fed into the transformer in-phase and in anti-phase. So you should not hear yourself on your own speaker. The same transformer also puts the signal on the phone line.