we don't specifically support choosing the sample rate for the mp3 encoder. we provide a front-end for LAME and it would sometimes crash the program when passing certain combinations of input and output sample rates to it.
your workaround is to install WavePad (http://www.nch.com.au/wavepad) and convert the sample rate there to what you want, and THEN do an encode to mp3.
btw kbps and kHz have no direct correlation - even though they both reflect file quality to a degree, there is no sense of "converting" kbps to kHz and vice versa.
