It's tough with an iPod touch, because there aren't any dock connector devices currently available that support simultaneous audio input and output (if someone is aware of one, please let us know). The problem with the iPod touch is that the headphone jack is next to the dock connector, so most of the dock connector input accessories block the headphone jack.
Your best bet might be to work with the Belkin Tunetalk and the SendStation dock extender
, plus an output adapter cable with a right-angle stereo mini plug. That way you can connect a measurement microphone to the input of the Tunetalk and connect the headphone jack of your iPod to your test speaker at the same time.
Also, I do not recommend using the ProTrack's or the Tunetalk's built-in microphones for acoustical measurements--they are not omnidirectional.