I found this text here.
As I understand it, Sound-juicer uses gstreamer to invoke LAME for mp3s. Unfortunately, it invokes LAME with some pretty low-quality settings by default as has been beaten about on many posts. I finally began to get true VBR mp3 files with fairly high bit rates by changing the mp3 profile:
click "Edit Profiles" at the bottom
highlight "CD Quality, MP3"
Replace the default Gstreamer pipeline with
audio/x-raw-int,rate=44100,channels=2 ! lame name=enc mode=0 vbr=4 vbr-quality=0 quality=0 vbr-min-bitrate=32 vbr-max-bitrate=320 lowpass-freq=20500 ! id3v2mux
close the edit
close soundjuicer and restart
edit/preferences and select "CD Quality, MP3"
close preferences and (hopefully) get halfway-decent mp3s.
Here's a guide to the pipeline command and its LAME equivalent
mode=0 -m stereo (change to mode=1 for joint stereo)
vbr=4 --vbr-new (use new vbr algorithm)
vbr-quality=0 -V 0 (use highest vbr quality , 9 is lowest)
quality=0 -q 0 (use highest quality encoding algorithm)
hopefully the rest are self-explanatory. I hope this helps someone.