As Koz wrote, it is very common for different devices to run at slightly different rates. Professional studios can get round this by using (expensive) hardware that can all be connected to one common “word clock” (http://en.wikipedia.org/wiki/Word_clock)
Provided that both the computer and the microphone are running at consistent speeds (the speed does not vary over time), then it is easy to accurately stretch an audio track to a precise length (in seconds) using this plug-in: http://forum.audacityteam.org/download/file.php?id=10288
For more information about the plug-in (and another more complex version), see this topic: https://forum.audacityteam.org/t/change-speed-stretch-audio-to-a-specified-length/23332/1