You have a few problems to solve:
- how you decode the output from the receiver to determine what has been sent
- if you use audio tones you will need some kind of decoder. It can be done in software but I don't know of any arduino code to do it. Have a look on google as per my earlier post. You could also use an external audio tone decoding chip, there are different kinds available. DTMF is perhaps the most common because its used in phones. You can do some of the testing of the audio decoding without requiring a transmitter/receiver.
2 . How you select the transmitter/receiver
- If you use something like an xbee pro you can get a few kilometers line of sight. This will be much easier to decode and its legal!
Transmitters use without a license is regulated in most countries for good reason. A badly tuned transmitter emitting 3 watts can cause a lot of interference, even if its only on for a few seconds at a time.