You can enable the Audio Search feature in File > Options > Audio and Video.
With the Audio Search feature enabled, you can search your audio and video recordings for words, just like you would search for typed text in your notes. For example:
Record phone conversations with customers, and later search for keyword of topics that you discussed
Record an interview and then search for specific quotes
Record voice reminders on a Windows Mobile Smartphone or Pocket PC. After you synchronize them with OneNote, you can search through your reminders.
The audio recognition process and indexing happens only when the computer is left idle and OneNote is running. It will take at least twice the length of a recording, so you'll have to let OneNote index for some time before you can begin searching.
When Audio Search is enabled, use the regular search box (above the page tabs) to type your search term and then click View List to see the audio results at the bottom of the results pane.