When I first started doing screencasts – videos of the screen recorded using the same device – I quickly discovered that the built in mics wouldn't do it. The problem with recording audio with a built in mic on a device you're tapping the screen on is that the mic will pick up every single tap and make it sound like there's an earthquake or something.
The solution to this problem came about in probably less than 5 minutes, but has been working ever since. I found a wired headset that came with either my Galaxy S II or Galaxy Tab 7.0 Plus (not sure which at this point), cut off the headphones, and removed the plastic from the mic. A couple of years ago I bought 10 Steelseries microphones on a huge sale for 95% off, and still have a few around in their original packaging. I ripped off the internals from one of them so that I got the casing with clip and wind shield, and simply taped the thing up. As I said, extremely quick operation, but the resulting mic has served me well ever since. It works on both Android and iOS, for screencasts and normal video recording. It's not a high tech mod by any means, but it allows me to strap a microphone in a better position that either the built in mics or any headset mics are in.