I own an iRiver, but it's a T-10 and not an iFP-799. My baby comes with a built in microphone and while the resulting audio file isn't perfect by any stretch, it's good enough for quick, off-the-cuff stuff.
However, if you're the proud owner of the iFP, Rich over on Geekfishing has done all the hard work for you. He's figured out the best way to set it up for recording, what microphones work best, and he's done it all with a bit of humour.