[Audacity/ffmpeg] Audio duration unsync

[butterw]

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.

[ProWo]

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.

[butterw]

There is no reported audio delay in the mp4.

Audio.m4a
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"

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

[ProWo]

Then 00:07:58.491 is the real duration. You can lengthen the audio track or shorten the video track.

[butterw]

Using the Change speed... Effect in Audacity I stretched the audio duration to match the video duration, which gave an acceptable result.