Not really sure about triggering audio as a default. You can map notes to a button, but I can't see a setting to make audio the default (only writing or typing).
You could try a 3rd party app. Vito notes has a good reputation, and costs anout $15:
http://vitotechnology.com/vito-audionotes.html
There is another freeware app that seems pretty cool...you may want to check it out:
http://handheld.softpedia.com/get/Au...es-42629.shtml