$ lame -b 48 -m m -q 2 in.wav out.mp3
produces a file with the desired characteristics for me:
$ file out.mp3
out.mp3: MPEG ADTS, layer III, v1, 48 kbps, 32 kHz, Monaural
$ file in.wav
in.wav: RIFF (little-endian) data, WAVE audio, Microsoft PCM, 16 bit,
stereo 44100 Hz
If the input file is not sampled at 32 kHz, lame will do the
resampling -- unlikely to be of the quality that Eric's or Fons'
resamplers provide, though at that target bit rate you might not
care too much :).