Werner - Don't use the options in 'Video' drop-menu.
In the left panel, you will have "MPEG-4 ASP" selected.
Just below that, is "Configure" - open that, and under "Encoding Mode" - Select "2-Pass Average Bitrate". That will give a good conversion with the audio in sync.
With the 2-Pass mode, on the first Pass, Avidemux "maps" the video, including parts with most and least changes in detail and colour, and the line of audio. This data it saves to a text-file - and on the second Pass - uses the file as the "what's ahead" information for correct encoding. If 1-Pass mode is used - Avidemux can only "guess" from where it is in the encoding, as to what the next part might be...
In that Configure dialog-box leave Motion, Quantisation and Rate at "default" - as you learn Avidemux these can be good presets if you know how they change the process. The "defaults" work quite well with Average Bitrate.
2-Pass Average Bitrate will default to 1,000 kb/s - you can increase the bitrate to raise quality - but it takes quite large increases to achieve much "visible difference" - and that means large file-sizes.
Note that increasing the bitrate of low-resolution video can't improve it much if at all. A video frame is similar to a JPEG image for info storage. If a friend mails you a 640 x 480 JPEG - 'enlarging' it to 1600 x 1200, even with best interpolation, won't do it much good at all...
Increasing bitrate in video can't "invent and insert" information that isn't in the original.
Below "Configure" is "Filters" - open that. Ensure that "Lock Aspect Ratio" is ticked, and both Source and Destinaton say "1:1".
Under "Resize Dimensions" - just leave the "default" (current frame-size) - unless you actually want to re-frame-size the video.
Use the Slider or the Percentage Spinner to its right to change the size. I'm assuming that you want to down-size camera video. Trying to up-size - unless by a very small amount - seldom has desirable results.
If you are downsizing to send to friends to play on PC, or to put into a video editor - you needn't use the "Round to nearest multiple of 16" check-box - 'unless' your video editor demands "compliant AR" (aspect ratio.)
If so - or, if you are going to make a correctly compliant MPEG2 DVD fileset - you do need to have the width and height as multiples of 16 for most applications.
Example - to convert 1280 x 720 camera Std-HD to standard DVD at 16:9 Aspect Ratio - with "16 multiples" selected, the frame-size will be 720 x 400 (or 404 - can vary with source-files) - and you'd need to use the "Add Black Borders" top and bottom - for a DVD to play correctly on a std 4:3 AR TV.
(The black borders will be different for US std TV 720 x 480, and PAL 720 x 576.)
The X/Y error should be 0 and 0, at 16-multiples - for compliant format.
For widescreen TVs - some are 16:9 and some are other ARs - you'd need to allow for that - unless you have a high-end DVD player or TV that adjusts automatically. (Friends here have a late model Panasonic that does.)
Lanczos3 is the best interpolation mode in Avidemux, though a bit slower than Bicubic.
Xvid - use the Configure dialog, not the menu drop-down. In there, select "2-Pass Average Bitrate" - unless you need to control output filesize - say, to fit on a CD - it will then show a 'Target Video Size" - 700MB, 1 CD, which you can alter.
You can also use Target Size to resize a movie - 4.6GB, 5.4GB, etc, to fit on a DVD5 single-layer, if you don't have, or don't want to use, a DVD9. A DVD5 is 4.3GB - but resulting filesize can be a little bigger than set as Target.
To ensure your, say, 5.8GB movie will fit a DVD5 - Target for 4,000MB (that's a bit under 4GB, as there are 1,024MB to 1GB) - and get a result between 3.9 and 4.2GB.
Using 2-Pass Average Bitrate for Xvid, the default bitrate will be 1,500 - you can lower that if needed - but as Xvid4 is a bit more efficient than std MPEG4, the output at 1,500 won't be much larger - and will be better quality - than MPEG4 at 1,000 bitrate.
With Xvid you can use MP3 for audio.
Video Editors will usually alter inserted video's audio to suit the rendered output - say, to AC3 or PCM for DVDs, etc.
You can, however - do a "Save" of the original video's audio track, with Avidemux.
Make sure the video is in detected and playable mode in Avidemux.
On the bar - click Audio > Main Track - and the dialog box that appears, in the centre bar, offers "Track from Video". Some movies have more than one sound-track - maybe AC3 and MP3 - in your case, select the AC3, and click OK.
You can also save the audio from your camera videos - as "add-back later" in editing, or to use some "original" - passing train, maybe - and some add-in music.
I have also heard that "some kids" use Avid to "grab" the tracks out of material from a 'tubular' sounding source - but that might not be polite in some regions, so one can't at all suggest that anyone do such....
Regards, Dave.
) I'm color blind and I cannot see the crappy captchas on the search page 
. Please implement captchas for the color blind too!! I got my kid to read the captcha just to register 
