I like to say there are two ways to explain VBR vs CBR. There is the non-tech way to explain it and that is simply:
VBR = Bad, CBR = Good
What we recommend at Libsyn is:
64 kbps - bit rate
44.1 khz - sample rate
mono or joint stereo.
Personally - I go with 96 kbps on the bit rate and mono.