From AmoK - The Art of Coding:
AmoK MP3 ReEncoder features re-encoding of MP3 to MP3 with fixed bitrate between eight and 320 kbps, re-encoding of MP3s with variable bitrate between V0 and V9, conserves MP3 meta data (ID3 tags), MP3, Flac and WAV as input, and free English support via email. AmoK MP3 ReEncoder offers all required features. It can process single files or thousands of them (and read in complete folders including subfolders). Due to its support for multi-core CPUs, several MP3s can be re encode at the same time.