[Vorbis] Encoding using Fishsound + Vorbis = Strange "RoboCop" artifact

[Bob Ingraham]

Hello Fellow-Vorbisites,

I've written a multi-track encoding/decoding library based upon fishsound.

Utilized Visual Studio 2010 to build 32-bit DLL's (Debug mode - no
optimization) of all the following:

libogg 1.3.1
libvorbis 1.3.3
libspeex 1.2rc1
libflac 1.2.1
liboggz 1.1.1
libfishsound 1.0.0

Currently testing under Windows 7 (64-bit).


Test Scenario:
==========
Recording from two microphones simultaneously, creating two mono tracks in
a single ogg file.

Using the exact same code to test fishsound with vorbis, flac, speex

The only variable that changes is the FishSound->format that is passed
into fish_sound_new().


Results:
======
If I use speex or flac, everything sounds sounds normal upon playback
(using Audacity for playback.)

If I use vorbis, my voice is recognizable, but I sound like a "robot" -
some strange artifact is introduced that adds this "RoboCop" effect to my
voice.

Samples can be downloaded at: http://tablet.brin.com/


Question:
=======
I'm trying to isolate the issue to either libvorbis or in libfishsound
(how it calls libvorbis in vorbis.c).

Any ideas on what would cause this artifact?

Thanks,
Bob