Recently I discovered gnump3d. It supports downsampling, which is a feature I especially need. However, the example reencoding command for handling MP3s doesn’t seem to work. Instead, I am using a different variant.
downsample_medium_mp3 = /usr/bin/sox -t mp3 $FILENAME -t wav - | /usr/bin/toolame -b 128 - -
The original uses the lame encoder to both read the MP3 source file and then reencode it, but that seems to fail miserably. Fortunately, using sox seems to resolve it.