The Speex docs say that it's 'mainly' designed for 8/16/32kHz sampling rates. Most PC inputs seem to report sampling rates of 8kHz, 16kHz and 44.1kHz.
Does this mean I'm out of luck if I want to use a 44.1kHz sampling rate?
Speex can handle any sampling rate (I have tested from 4kHz up to 48kHz, but Speex can do probably even higher), but it is optimized for 8 (narrow band), 16 (wide band) and 32kHz (ultra wide band). For my application, 24kHz (using wide band) sounds good (I find it better sounding than 32kHz at the same bitrate, but this depends on the audio material and taste).