Most MP3's that you'll find through file sharing networks employ a Joint Stereo audio track. Joint Stereo is an attempt at reducing the size of a file while still retaining "stereo" signals. In a joint stereo file, the compressor takes advantage of the fact that both stereo channels contain mostly the same information most of the time. So, when the two channels are similar, instead of compressing both a Left and Right channel, it adds the two together and produces a middle (L+R) and a side (L-R) channel. This allows a reduction in the final size of the file by using less bits for the side channel. During playback, the decoder will reconstruct the left and right channels from the middle and side data.

Sounds great, right? Well, not really, because when the middle and side channels are created, a lot of information from the original Left and Right channels is lost. This lost data generally ends up being subtle harmonics and tonal depth, and without it, you end up with a flat sound.

But not just yet; there's so... much... 80s music! So much downloading to do!

UPDATE: Aw hell... there's the e-mail invoice for today's music purchasing frenzy.

$10.89.

For what would normally have cost me around $50 in whole CDs, most of which I didn't actually want, plus gas and time.

UPDATE: Chris has a few more details:

The 'joint stereo' is usually applied to low frequency effects, because the ear can't tell whether it's coming from the left, or right, as much as middle or higher frequencies. This is why you can put your subwoofer off to one side and still have a good stereo environment.

However, the key is the 'as much' part. It's not a straight cutoff, but a lowering of the spatial perception as the frequencies get lower. I'm sure different people have different cutoffs, too, so the whole 'joint stereo' thing is an approximation; much better to just KEEP them separate, if you have the bandwidth to spare.