I am guessing that you have multiple Mp3 files and your own video files to go with the music. If you are ripping the files from cd then try a different constant high bitrate mp3 or best of all as Marc says convert or rip to wav. The lame mp3 codec can cause problems when being imported in to audio/video apps that are using the Fraunhofer official codec.

AV sync problems can be a nightmare and it is rare that the fix is quick.

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Edius Pro, ADVC300