First off guys.. YOU MUST SEARCH and read threads before posting your problems with CS4.. There are more Threads here in regards to people updating from v4.0 to v4.0.1 and losing the AC3 Audio. Clearly if we were to read we would see that MANY off us have encountered the same problem and many users have posted TEMP solutions...
If you have recently updated PPCS4 from v4.0 to v4.0.1 and you have lost your audio revert back to version 4 (by uninstalling PPCS4) and reinstall the program from scratch. This solution will solve your problem..
Adobe are fully aware off this and are working on a fix (so I'm told) when your guess is as good as mine...!
For all those who have never had audio with v4.0, then try either connecting your speakers or turn the dam things on as suggested by Harm Millaard.....! Many thanks...!