Quite a lot of people have been having problems with low audio, or pop's and clicks. The audio can be improved during conversion using the most popular programs.

Virtual Dub
Audio > Full Processing mode
Audio > Volume

This can be used to increase the volume. If you intend keeping the movie as an AVI you now need to save out another copy using full processing mode.

Video > Full processing mode
Video > Compression > Codec > Bitrate (Choose codec to match your source)

Audio > Full processing Mode
Audio > Compression > Codec > Bitrate (Choose codec to match your source)
Audio > Volume > Increase

SAVE AVI
This will resave the AVI compressed, with an increase in the volume.

TMPGEnc
The same can be done using TMPGEnc, and this is the most popular way, when converting AVI to VCD.
Setting > Audio > Use audio edit > Setting > Change volume
You can experiment with different settings, and click the Play button to test the level. You can also use the normalize button to scan for the highest and lowest peeks and set the audio at its average level, this will increase levels that are low, and decrease levels that are high.

GoldWave
Another method of increasing the level, or fixing pops and clicks, is by loading the audio into a sound editor. My favourite Sound editor is one that has been around for a long time, and has since been copied by many.
GoldWave was the first commercial sound editor that gave the user the ability to use flange, fade in/out, robot and a whole lot more. Goldwave can be downloaded as "trialwear" from www.goldwave.com (Also Download MP3 plug-ins)

Using Goldwave is pretty much straight forward, even the first time users will quickly learn how to use its many enhancements. GoldWave will except AVI as the source, so it is not always necessary to extract the audio before using it.

Load the AVI or a previously saved WAV into Goldwave.

Typical compressed MP3 wave form (Low level) Before change


Select Effects > Maximize > Select new RMS. You will see the current RMS value, and can experiment with different levels, double the figure it shows, for a 100% increase. In this example I have used the default of (0.2000)

Typical compressed MP3 wave form (Maximized at 0.2000) After change


This can significantly improve the audio level without distortion, There are many filters for use, that can also remove annoying hiss, clicks, and buzz. You can also use this program to change a mono sound track to stereo, simply save as stereo to convert.

You can also swap channels by clicking the channel toggle icon. You may notice one of the channels slightly different to the other. This is normal if the left speaker might have a sound effect not wanted in the right speaker.

Once you have finished editing you can save out the track to a WAV or MP3 for use later. If you intend using this with TMPGEnc to make a VCD, you may want to use 44.1Khz 16 bit stereo (Uncompressed)

When saving as MP3 make sure you match the playback rate, as this can speed up or slow down the sound :)

Update, Missing Audio
There have been a few AVI's with missing audio at the end, reports are about 6 seconds, Those 6 seconds of "Blank" do not have a silent audio stream, so the audio is missing.
Any attempt to save video and audio, would then give a big difference in length (In this case 6 seconds). If you see a big difference when loading the stream, then you will know you have the missing audio stream.
Vdub MP3
Go to the end of the AVI, and come back a few minutes then press play, press stop as soon as the audio is not heard, then click the keyframe back once and mark that as start. Then move to the end of the movie and mark that as end. Delete those frames and save out another copy using video/audio direct stream copy. You have now removed the frames that have no audio, and can reload that back to work on as normal