Open your video with virtualdubmod.
http://sourceforge.net/projects/virtualdubmod/
Note that Virtualdubmod does not like variable bit rate audio, so you may have to convert the audio first. If you get a warning about variable bitrate then you have to convert the audio first to CBR. If you didn't get a warning, then the video is already CBR and you can skip the instructions about converting audio and go to synchronizing the audio.
CONVERTING VBR to CBR
When it asks if you still want to rewrite the header, click on NO
Go to STREAMS -> STREAM LIST
Right-click on the audio stream -> FULL PROCESSING MODE
Right-click the audio stream again -> COMPRESSION
Choose LameMP3
(You may have to install lame first)
http://users.tpg.com.au/mtam/install_lame.htm
Pick your bitrate, e.g. 48000Hz 128 kbs CBR
OK, OK
VIDEO - DIRECT STREAM COPY
FILE - SAVE AS...
After it has finished, close Virtualdubmod.
Now open the new (CBR) video file with Virtualdubmod
SYNCHRONIZE THE AUDIO
click on streams
right-click on the audio stream
choose interleaving
Choose the amount of time you need to delay, (6000ms)
OK
OK
You can preview the video in virtualdubmod to see how the timing is. If you need fine tuning adjust the delay as above.
Now, click on VIDEO -> DIRECT STREAM COPY
FILE -> SAVE AS...
That's it.
Be sure to click on "video -> direct stream copy" before saving. If you leave the video in full processing mode (default) without choosing a compression, it will give you a humongous uncompressed video file that may fill up your entire hard drive. You can preview the video while full-processing mode (default) is checked so you should wait until you're ready to save your file before changing it to Direct Stream Copy.
Also note that you don't need to use full processing mode to resynch the audio (interleaving). You do need to use full processing with compression to change the audio from VBR to CBR.