This article is from the CD-Recordable FAQ, by Andy McFadden (firstname.lastname@example.org) with numerous contributions by others.
Some discs have been produced that call themselves "AVCD", as in
audio-video CD. For example, Kylie Minogue's "Fever" CD was released
as a two-disc set in Asia. Disc one was the "Fever" audio CD, disc
two had four VideoCD video tracks and five bonus audio tracks.
If you put disc two into a CD player, you would hear nothing for track 1
(which holds the VideoCD filesystem) or tracks 2 through 5 (the video
data). If you fast-forwarded to track 6, you would hear music.
If you put disc two into a VideoCD player or compatible DVD player,
you would be treated to the first video track. By skipping forward
you could get to the later video tracks and eventually play the audio
This makes perfect sense until you try to figure out how the same audio
track is being played on a CD player and on a VideoCD player. If you
try to create a VideoCD with extra audio tracks, the VideoCD player
will not find them.
The trick used by the AVCD publishers is to encode the audio tracks twice.
The songs are present both as Red Book CD audio tracks and as VideoCD
compressed audio. A directory called "CDDA" holds files with names like
"AUDIO06.DAT" that contain compressed audio. Unlike the video tracks,
these don't actually correspond to tracks on the disc.
To create such a disc, you would need VideoCD authoring software capable
of incorporating audio tracks. You could then record the VideoCD while
leaving the session open, and append the audio tracks using track-at-once
recording. Better results would be obtained by writing the video and audio
tracks with disc-at-once recording, but that might require a greater level
of VideoCD support than most recording applications currently provide.
See section (3-16-1) for more tips on VideoCD.