I think this is a “video file problem”. The more-compressed audio/video files can go sometimes out-of-sync when editing or converting (MPEG-4 seems to be worse than MPEG-2.) There are time-codes in the video and they can be fouled-up in a way that the original plays OK, but it gets messed-up when edited/converted.
I’ve never used iMovie, but if it can export to a less-compressed audio/video format it may stay in sync and you may be able to extract and then re-combine the audio without it going out of sync. Sometimes I’ve solved the problem by simply saving/exporting in the same format. (If it’s lossy compression, that’s an extra lossy-compression step but it’s worth it if that solves the problem.)
MP3 adds a few milliseconds of silence to the beginning, and possibly the end. So, the MP3 is actually longer than the original uncompressed audio. If you re-multiplex the audio & video, the audio will lag but it should be a constant-lag and you should be able to adjust it in your video editor.
Not sure why it is shorter, but I did not change any sample rate at all in Audacity, yet it says on the lower right corner the actual rate being 44800, instead of 44100… So how do I fix this…?
Most video-sound is 48kHz. CDs are 44.1kHz. MP3s ripped form CD are usually 44.1kHz and MP3s ripped from movies are usually 48kHz. Changing the sample rate in Audacity doesn’t change the playback time/speed. The audio is re-sampled so the tempo & pitch don’t get messed-up.
So… What are you really-really trying to do? Why are you converting it to MP3, then to AIFF, and then… what? Are you trying to put AIFF (or uncompressed LPCM) back into the video file? It looks like you’ve got a couple of extra steps, and I don’t understand why you’re converting to MP3.
Thank you DVDdoug. I was always exporting the video or audio files from iMovie in order to send them out through emails or upload online. The only files I was exporting were mp4 or mp3. I was so used to go autopilot to choose either mp3 or mp4 for either video or audio, without thinking of the compressing fact. So…my big No No causing me this project. But anyway, now I am aware of it; too late but… oh well…
Another question, would there be any distortion or out of sync if one file with diff then to wav… I am actually using both windows and apple between programs…I know; it sounds like extra work and unnecessary, but I cannot use my trackpad editing Audacity as good as a mouse with a window desktop…