Hi, I am trying to use `mp4mux` to multiplex into a single MP4 file:
- a pre-fragmented MP4 file carrying an unencrypted AAC audio track (audio.mp4)
- a pre-fragmented MP4 file carrying an unencrypted AVC video track (video.mp4)

Each input file was obtained by mere concatenation of a dash init and a dash media segment from an unencrypted live DASH stream.
The output file's `ftyp` and `moov` box seem consistent.
However, despite the presence of audio and video (playable) media data in the input files, the output file's `mdat` box remains empty. 

Is `mp4mux` lacking information about input samples ? Should I enrich the audio/video init segments with some information about the actual number of available samples to make this work ? (the `entry_count`, `sample_count`, `duration`, ... fields from input init segments are set to 0 because they are not known for live streams). Is there a simpler way to do this ?
Thanks in advance for any guidance.

```
$ mp4mux --verbose --track video.mp4 --track audio.mp4 out.mp4
MP4 Import: importing all tracks from video.mp4
MP4 Import: importing all tracks from audio.mp4
$ mp4dump video.mp4 | grep mdat
[mdat] size=8+427457
$ mp4dump audio.mp4 | grep mdat
[mdat] size=8+23475
$ mp4dump out.mp4 | grep mdat
[mdat] size=8+0
```


https://user-images.githubusercontent.com/5099987/158215819-1b4a78d5-ebad-48aa-a035-003100551711.mp4


https://user-images.githubusercontent.com/5099987/158215823-cb692d09-e72b-4030-8331-cc6707285f26.mp4


mp4mux outputs empty mdat boxes
This is a GitHub Issue
repo:Bento4
owner:axiomatic-systems
Title : mp4mux outputs empty mdat boxes
Issue date:

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Hi, I am trying to use `mp4mux` to multiplex into a single MP4 file:
- a pre-fragmented MP4 file carrying an unencrypted AAC audio track (audio.mp4)
- a pre-fragmented MP4 file carrying an unencrypted AVC video track (video.mp4)

Each input file was obtained by mere concatenation of a dash init and a dash media segment from an unencrypted live DASH stream.
The output file's `ftyp` and `moov` box seem consistent.
However, despite the presence of audio and video (playable) media data in the input files, the output file's `mdat` box remains empty. 

Is `mp4mux` lacking information about input samples ? Should I enrich the audio/video init segments with some information about the actual number of available samples to make this work ? (the `entry_count`, `sample_count`, `duration`, ... fields from input init segments are set to 0 because they are not known for live streams). Is there a simpler way to do this ?
Thanks in advance for any guidance.

```
$ mp4mux --verbose --track video.mp4 --track audio.mp4 out.mp4
MP4 Import: importing all tracks from video.mp4
MP4 Import: importing all tracks from audio.mp4
$ mp4dump video.mp4 | grep mdat
[mdat] size=8+427457
$ mp4dump audio.mp4 | grep mdat
[mdat] size=8+23475
$ mp4dump out.mp4 | grep mdat
[mdat] size=8+0
```


https://user-images.githubusercontent.com/5099987/158215819-1b4a78d5-ebad-48aa-a035-003100551711.mp4


https://user-images.githubusercontent.com/5099987/158215823-cb692d09-e72b-4030-8331-cc6707285f26.mp4


--- end body ---
