Nikon D3100 Video Mic?
The hidden message in cman548's reply is that good audio is as complicated as good video. Doing it on the cheap is pretty difficult (though the maxim that "a bad mic well placed is as good as a great mic poorly placed" holds true). There's really no such thing as good cheap wireless audio, unless by cheap you mean over $300. And most audio people collect mics the way photogs collect lenses. Each has its strengths; the best mic depends on the environment in which it will be used. I'd investigate a wired system. AudioTechnica has a good range of products ranging from cheap-but-passable $30 lavaliers to pro-quality systems. The cheapest approach for decent audio in an indoor is to buy a Zoom or Tascam recorder and either two lavaliers or a shotgun and run it directly into the recorder. If you go with a shotgun you'll need a mic stand or a helper. Whatever your choice, do a LOT of reading on the equipment and how best to use it before buying. Some mics are phantom powered, some have batteries; some directional, some omni directional. Each have optimal positioning, etc. The basic rule of thumb for good audio, though, is get the mic as close as possible to the talent.