LOTS!!! There are several problems with your circuit and what you want to do (which may be ILLEGAL!).
Aircraft audio systems are very different from civil/domestic systems. The first thing is that ALL aircaft headset mics and handmics are POWERED types. The mic input circuit of the transmitter supplies a 'bias' voltage of 6V to 8V dc to the mic connection on the plug. This goes back to the days when aircraft mics were carbon, like in old telephones, hence the powering voltage. If you look at the schematic of the microphone part of the headset you will see a right-pointing triangle - this represents an included pre-amplifier, powered from that voltage. You cannot connect your ipod (or whatever) output directly across the mic line as the low output resistance of the ipod will seriously interfere with the amplifier circuit, preventing the aircraft microphone from working (which you wouldn't want to do!) while you also risk damaging the output circuit of the ipod (or other device) by connecting this voltage to it. Note that when not transmitting, this voltage may rise to full rail, depending on how the PTT is arranged. A diode won't help.
Next problem: Aircraft radios have a headphone output typically sourced from a couple of hundred ohms. If you measure the dc resistance of your headphones, you will find it varies from a few hundred ohms to over a thousand, depending on the position of the volume control. If you were to make a parallel connection across this circuit, as you have shown, the low-impedance ipod output circuit will (again) effectively "short-out"
the higher impedance radio output, dropping essential comms volume to the headphones and may cause distortion as well.
In short order, your circuit will NOT work in any successful manner, as you will now see, and could compromise safety of the aircraft.
CAUTION: I have to make comment on your intention to "broadcast" messages in this manner. Firstly, International Aviation Regulations clearly PROHIBIT "external" or "adapted" connections of any type to aircraft radio systems, unless they are approved by the CAA in that country, and secondly, the intention to broadcast prerecorded messages IS, unless approved, contrary to CAA regulations, period.