Introduction: Make a Stereo Microphone for $15
Example of the sound quality you can get out of this:
Step 1: What You'll Need
1 Female RCA to Male 3.5mm Cable:
2 Male RCA to Female 3.5mm Adapters:
2 3.5mm Microphones:
As user Rogue Agent pointed out in the comments, you should take a look at the frequency range of the microphones you're buying. The ones I linked to are made for voice recording, so they aren't the best for recording music/sound. Human hearing range is about 20Hz - 20kHz, so microphones closer to that range would work better.
Step 2: Put It Together
There really is no wrong way to do this.
Protip: Use the picture as a guide.
Step 3: You're Done. Now Put It to Use
Well, this instructable went by pretty fast. Probably because the whole "assemble it" part was incredibly simple.
Anyway, enjoy your new stereo microphone. Use it in place of your camera's horrible sounding built in one, or to record ambient sounds. Did I say that already? Oh, whatever.
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Please be positive and constructive.