Paul and all,
I have injected composite audio into mini transmitters like this that use the ROHM FM transmitter chip. The datasheet can be found in several places, including here:
http://media.digikey.com/pdf/Data%20She ... H1415F.pdf
Before I say how, first please note this
BIG-TIME DISCLAIMER: 1) at best this may not work, 2) at worst, it may irrecoverably blow up your equipment, 3) it will require soldering work at a component level, including some parts removal. 4) it will void all warranties, and 5) see # 1.
Now that I've sufficiently scared you, compare the chip's internal workings in the datasheet above to the transmitter's schematic. You'll see that the internally-generated composite audio (the one that you don't want) comes out of pin 5, goes through some circuitry and then is re-inserted into pin 9 for modulation.
Exactly what that "some circuitry" does, and where the internal pilot is injected or turned off is unclear. In the older versions of this chip, I would merely lift the right side of R2, the bottom side of R20, connect those two lifted points together, and pull the 38 KHz crystal (to kill the internal pilot). Composite would then be sent into one side of the input jack.
This version appears to generate the pilot internally via division of the master clock, and they may be doing some RF work with D1 (AFC, VCO or both). Thus for this unit I would suggest lifting the top side of R17 (versus R20) and connecting R2 and R17 together. Start with your composite audio at an *extremely* low level, and increase slowly.
Again it has to be said: there are many steps and points where this can all go very wrong. It helps just a bit that I have a spectrum analyzer, modulation monitor and am a broadcast engineer.
But... if you're willing to blow up your unit in the interest of science...