Vidmex Wizard allows to extract audio fragment from avi file. Just mark the fragment, and then push "audio fragment" button.
First, if you want to extract audio as MP3, choose *.MP3 file type, or choose *.WAV for wave file accordingly. This is very important, because extension gives information on how to use audio file. Secondly, after that you will see the next dialog. Choose some codec, which will be used to extract static video fragment with sound from avi file. Thirdly, choose type of file PCM (Wave) or MPEG Layer-3 (MP3), the same as file extension you want. Here you can also specify frequency (quality) of sound.

So, common steps to extract mp3 are:
1) Audio fragment
2) Save as... File type *.MP3
3) DivX codec
4) MPEG Layer-3 type, frequency 44Khz

In order to extract wave follow these steps:
1) Audio fragment
2) Save as... File type *.WAV
3) DivX codec
4) PCM type, frequency 44Khz

NOTE: Audio operations depends on the audio codecs installed on your system. You can save MP3 fragment only if your system supports MPEG Layer-3 codec. Also, don't use AVI files with preload skew: 1 sample.