I'm not sure whether there are any audio rippers, but if you just want to grab the audio, you can just use any sound recording program (I'm pretty sure most OS's come with one pre-installed) and record the sound-out while playing back the AVI.
You may have to tweak the sound recording settings in your volume control manager (some systems ignore sound output when recording -- if for some reason your system prevents you from changing that, or you can't figure it out, just get an inverter cable and bridge the main sound output (e.g. headphone) and sound input (i.e. microphone) slots of your sound card).