The problem may be that 11kHz, I believe, isn't a valid mp3 sample rate. 32, 48, and 44.1 kHz are the only valid rates I'm aware of. But let me see if I can find out something else. There has to be something somewhere that claims 11kHz as a valid sample rate, as mp3s don't record the actual sample rate in the file; they just record a dictionary entry.

Edit: Yup. Every doc I can find has the same info as this one, and it says that 11kHz is not a valid sample rate, although maybe if it was MPEG 2.5 Layer 3, a sample rate of 11025 Hz might get reported as 11kHz, but MPEG2.5 doesn't exactly seem likely.