Both the PowerBook 1400 and PowerBook 2400 computers use a 1v Peak-to-Peak, line level, microphone. You can find these types of microphones at almost any electronic stores. One such microphone found at Radio Shack is the Sound Level Meter (Radio Part Number: 33-2050). The cable we used with this microphone was also from Radio Shack (Radio Shack PN: 42-2481).
Note: The Radio Shack Sound Level Meter only records in mono. This means that with this particular microphone you will either record the left or right channel depending on which RCA cable is connected to the meter.