1. Encode the WAV into an MP2 file by leaving video source blank and choosing your WAV for the audio source. Stream type should be ES (Audio only).
2. Click File > MPEG tools
3. On the Simple Multiplex tab, choose your encoding method, video input (the *.m2v file) and audio input (the *.mp2 file), and output filename.
4. Click Run. And wait.
You might get some error when it finishes, but it should work. I did this to make a bleeped version of Signs.
Nathan