I've done this before in the past. I did the following:
- Get image of person
- Using Photoshop, cut out the mouth and then re-paste it into the image so it becomes a separate layer
- Run a desktop-grabbing progam (I use one called SnagIt - you can download a trial version off the internet, just google search for SnagIt)
- Back in Photoshop, hide everything on the screen (apart from the image and mouth layer) and select the "Move" tool (as if you would like to move the layer i.e the Mouth cut-out layer) around - this means you can move the mouth up and down to emulate talking
- Now, run SnagIt and choose to capture a "Region" of the screen as a Video (i.e AVI file)
- Whilst recording in SnagIt, use the Move tool in Photoshop to move the mouth up and down - it's useful to run your audio file via Media Player, etc at the same time so as to synchronise the movement with the dialogue
- Stop SnagIt when finished - it will export the footahe to an AVI video file
- Open your video editing software and crop the video sequence so it cuts out any extra stuff that you don't want (i.e you moving the mouse, selecting stuff, etc)
- Place your audio file into the timeline
- Export the final sequence as an MPEG, WMV, etc
- Done!
The above way is a relatively cheap solution, but I find it's quite effective - the trick is getting the mouth to move right on cue with the sound but that's more about timing than software limitations. I'm sure there's other tools that will do this, but this is at least one way to try.
Hope this helps.