I have a large number of Premiere CS3 files that contain imported MPEG video. After opening in CS4 the sound does not play (the audio track is still present in the timeline). The sound is still fine in CS3.
To import them into CS3 I had to copy the ad2ac3dec.dll file from Encore to the Premiere folder so I copied that to the Premiere CS4 folder without success. Neither can I import MPG with sound in CS4.