The highest quality free audio codec is known as FLAC. It is lossless and has no compression, so when you listen to the music you are able to hear all the instruments with clarity.
DivX itself is a video codec and it uses AAC as audio codec.
If you only have audio but no video, or video but no audio, you are most likely missing a codec required to play the file. You need to find out what codec the video was encoded with, then download and install that codec. You might also check if a newer version of Windows Media Player is available by checking the optional updates section of Windows Update.
The AC'97 codec was invented by computer giant Intel as a mean of decoding, recoding and producing high quality audio streams at a reduced processor usage.
You can find several video and audio codecs in the Combined Community Codec Pack, that you can find via google.
It would depend on the format of the audio file. However K-Lite codec pack should cover most of you bases. It is usually the only codec pack I install on a computer.
MPEG-3
MP3
Audio Codec 3 is a product of Dolby Laboratories.
Codec is a piece of software that translates video and audio data into and from proprietary formats of which MKV is one. It is the codec that enables the computer to interpret data from the proprietary source format.
Super I/O
ICH (Input/Output Control Hub
Actually ffdshow is a free software audio and video codec supporting several different formats. The application is only available on Microsoft Windows operating systems.