You need to make sure that the max bitrate in the MP3 Encoder settings is less than 160 kbps
If it's higher, then the LAME MP3 Encoder 3.97 (winamp\plugins\lame_enc.dll) can't make an mp3 (mpeg 1.0 layer 3) from an 8bit 11KHz WAV source file.
When the max bitrate is set to 160 or less, then you will see that the resulting mp3 will be an mpeg 2 / 2.5 layer 3 file (instead of mpeg 1.0).
This issue has been addressed in lame_enc v3.98 and newer, but it is still in beta stage, and therefore we can't include it in the Winamp distribution yet.
So basically, either change the Quality drop-down (Winamp -> Prefs -> CD Ripping -> Encoder tab -> MP3 Encoder) to: Normal, and the Mode to 'CBR', with a bitrate of 160 or less... or keep with the VBR setting, but make sure the max bitrate is 160 or less.
Alternatively, you can extract lame_enc.dll from the LAME 3.98 Beta package on rarewares, and copy it to the winamp\plugins folder (overwriting our 3.97 version), and then you'll find that the resulting mp3 will automatically be adjusted to the correct output format (mpeg 2.5 at less than 160kbps), regardless of your mp3 encoder settings....