There are two "black boxes" (usually a bright orange-red that's easier to spot). One is the cockpit voice recorder. While it's not that hard to record voice, you have to record it in a way that works even if the aircraft loses radio (so anything based outside the plane, such as continuous transmission, is not an option), that keeps working or at least doesn't lose data under water, that can transmit a localizer beacon to people searching for it ... Plus, you're not just recording voice. The recording has to be good enough to identify the sound of switches being thrown and other cockpit activity besides talking. All in all, this is not something you could do with a mike hooked up to your iPod, nor would you want to with so much at stake.

The other "black box" is the flight data recorder. It provides a continuous record of aircraft operating parameters, including the positions of all the controls, velocity/altitude, and more. The FAA requires recording at least 11 parameters on smaller passenger aircraft, up to about 30 on the big ones, but most FDRs go beyond that. Modern solid-state FDRs can record several hundred parameters for over 24 hours. Even if there were another way to handle the sound issue, you'd still need this one. There is nothing comparable to it on the consumer market. It is by far the more expensive of the two.

If there was a substantially less expensive way to meet all the functional requirements here, I'm sure it would be in use.