Thanks Mark for your prompt replies,
I think I'm starting to figure this out, but I'm looking at WaveMixerStream32 and MixingSampleProvider - Why do both require Ieee as the encoding? I'm using NSpeex and it appears to be using PCM. Speex also is 16 bit, whereas WaveMixerStream32 mixes 32 bit.
As an audio noob, what all would I need to do to mix this?
Thanks!