Here’s a circuit that provides some gain to iPod line-out audio to drive the TXD pin of a Radiometrix NTX2. It uses an LM386N audio amplifier chip (available from Radio Shack). I just substituted capacitors until it worked. Its powered by +5v DC from an Arduino pin.
Roughly based on this circuit – http://www.instructables.com/image/FTZUHWBGD2J36P8
Next will be trying the TS922 op amp.