[sdlsound] AIFF decoder bug

[Ryan C. Gordon]

> I set out to create some test cases for the audio converter, but got
> side-tracked when I found a bug in the AIFF decoder instead: It could only
> play mono, not stereo. This patch ought to do the trick.

Applied to CVS.

> About the test cases, does any of the sound formats we handle use unsigned
> 16-bit samples? I haven't managed to get SoX to create any of those yet.

Honestly, I've never heard of a sample that does so (I've only seen 8 bit
unsigned samples).

--ryan.