First, download a free app called Audacity:

To convert the mp3 file you have to WAV:

- Open the Audacity app.
- Use File > Open... menu to select the mp3 file.

Now you should see the waveform of the audio in a window.

- Use File > Export... menu to open the save file dialog.
- From the Format popup menu at the bottom of the dialog, select "WAV (Microsoft) signed 16 bit PCM"
- Click Save.

This should give you a WAV file that InqScribe can better estimate the duration of. The file will probably be bigger than the original mp3, but you should only need it while you transcribe. After that you can throw it away, since you can always create it again from the source mp3 file that you have.