There are two easy ways I know of for Windows users, involving free downloadable software:
AoA say their extractor turns AVI, MPEG, MPG, FLV (Flash Video), DAT, WMV, ASF, MOV, MP4 and 3GP video files into MP3, WAV and AC3 audio file formats. I've only tried WMV to MP3 so far.
Clearly it's a demo for their paid-for software - they have an arsenal of programs to do stuff with videos and audio files, even DVDs.
It's self-explanatory. Click Add Files, navigate to the movie files you want to add, and select a batch, or just one file if you prefer. Click Browse at the bottom to choose where on your hard drive you want the converted audio to be saved. When you're ready, click Start and go have a cuppa.
plus a whole bunch more like FLI, FLC, M2T, MKV, NSV, OGG, QT, RAM, RM(VB), STR, SWF, TS, TRP, VIV and VOB.
It claims to produce not just MP3, WAV and AC3 sound files but also AAC, AMR_NB (for ring tones), AMR_WB, MMF (for ring tones), MP2, MPC (MusePack), OGG and WMA, but I again haven't tried all the variations yet!
The software is free but you have to go through several pages of ads to get to the download link. I don't begrudge them that, and I do check out the ads there. I think it's only fair as the software is free. (I've blogged about Super before in my post on how to convert 3gp video from mobile phones and rotate the video 90 degrees - see that post for screenshot.)
ComparisonBoth programs let you convert a whole batch of digital films at one go, rather than having to extract the soundtrack from each one laboriously one at a time. AoA even lets you select and convert to audio just an extract or clip from a video, rather than the full soundtrack.
I tested the software when I converted the videos I shot at the London Girl Geek Dinner about search engine optimisation on 25 March 2008. I converted them to MP3 (hear the extracted audio on the same post), and both Super and AoA Audio Extractor did the job fine. I found that AoA was a little faster, and definitely more user friendly, while Super offers more choice of file formats it can convert to and from (see my tips on using Super - although written in a slightly different context, they should help as they still deal with file conversion).
You pays your money... or rather not, in this case, so why not try both out and see which suits you better.
Tip: before uploading the MP3s, I used the open source multi-platform audio editing tool Audacity to compress and normalise the audio files, which makes the speeches sound much clearer to the listener (I was recording with a digital video camera in quite a noisy pub environment).