Create fragmented MP4 from MP3

I am trying to convert an MP3 file to a fragmented MP4 like this:

ffmpeg -i input.mp3 -strict experimental -acodec aac -b:a 256k -f mp4 \
       -movflags faststart+frag_keyframe+empty_moov+separate_moof output.mp4 

However, using Bento4 I can see that there is just one giant mdat object instead of a series of those:

[ftyp] size=8+24
  major_brand = isom
  minor_version = 200
  compatible_brand = isom
  compatible_brand = iso2
  compatible_brand = iso6
  compatible_brand = mp41
[moov] size=8+701
  ...
[moof] size=8+62364
  ...
[mdat] size=8+5794679
[mfra] size=8+59
  [tfra] size=12+31, version=1
    track_ID = 1
    length_size_of_traf_num = 0
    length_size_of_trun_num = 0
    length_size_of_sample_num = 0
  [mfro] size=12+4
    mfra_size = 67

I think what I want is this:

(source)

But I can't seem to be able to get this from ffmpeg.

I found some other options here like

$ ffmpeg -h muxer=ismv
...
-frag_duration     <int>   E.... Maximum fragment duration
-min_frag_duration <int>   E.... Minimum fragment duration
-frag_size         <int>   E.... Maximum fragment size

but playing around with these didn't change the output.

How can I create fragments of a specific sice e.g. ~5 seconds each?

Audio streams have no keyframes which is what MP4 muxer relies on by default to demarcate fragment boundaries. You will have to set a fragment duration.

ffmpeg -i input.mp3 -c:a aac -b:a 256k -f mp4 -movflags +empty_moov+separate_moof -frag_duration 10M output.mp4

Note:

1. faststart is applicable for regular, non-fragmented output
2. frag_keyframe tells ffmpeg to start fragments at video keyframes but you don't have a video stream.
3. -strict experimental hasn't been required for the native AAC encoder since 2016.
4. frag_duration expects a value in microseconds. 10M = 10 million = 10 seconds.