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  1. To clean-up the audio of a mp4 video, I opened it in Audacity.

    However the audio duration in Audacity doesn't match the original duration, which causes unsync when I mux it back with the video after processing.
    original audio (m4a): 00:08:04.998
    after the file is opened in audacity v2.4.2: 00:07:58.491

    What could cause this and is there a way to get around it ?

    I tried extracting the m4a with ffmpeg, and also reencoding with ffmpeg before opening in Audacity but it doesn't make any difference.
    Last edited by butterw; 4th Feb 2021 at 11:58.
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  2. Do you have demuxed the audio from a movie? If you check the movie with mediainfo, is there an audio delay? Post this mediainfo report here.
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  3. There is no reported audio delay in the mp4.

    ID : 1
    Format : AAC LC
    Format/Info : Advanced Audio Codec Low Complexity
    Codec ID : mp4a-40-2
    Duration : 8 min 5 s
    Bit rate mode : Variable
    Bit rate : 126 kb/s
    Maximum bit rate : 128 kb/s
    Channel(s) : 2 channels
    Channel layout : L R
    Sampling rate : 44.1 kHz
    Frame rate : 43.066 FPS (1024 SPF)
    Compression mode : Lossy
    Stream size : 7.30 MiB (98%)
    Default : Yes
    Alternate group : 1

    When I reencode to mp3 the duration changes to 07:58
    ffmpeg -hide_banner -i "video.mp4" -vn -c:a libmp3lame -b:a 128k "audio.mp3"

    Format : MPEG Audio
    Format version : Version 1
    Format profile : Layer 3
    Format settings : Joint stereo
    Duration : 7 min 58 s
    Bit rate mode : Constant
    Bit rate : 128 kb/s
    Channel(s) : 2 channels
    Sampling rate : 44.1 kHz
    Frame rate : 38.281 FPS (1152 SPF)
    Compression mode : Lossy
    Stream size : 7.30 MiB (100%)
    Writing library : LAME3.99.5
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  4. Then 00:07:58.491 is the real duration. You can lengthen the audio track or shorten the video track.
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  5. Using the Change speed... Effect in Audacity I stretched the audio duration to match the video duration, which gave an acceptable result.
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