ffmpeg -i original_mulitrack.mp4 -vcodec copy -acodec mp2 -f mp4 new_multitrack.mp4

Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'original_mulitrack.mp4':
  Metadata:
    major_brand     : mp42
    minor_version   : 0
    compatible_brands: mp42iso2isomavc1
  Duration: 02:07:42.44, start: 0.000000, bitrate: 1664 kb/s
    Stream #0.0(eng): Video: h264, yuv420p, 720x404 [PAR 101:135 DAR 4:3], 1280 kb/s, 29.97 fps, 29.97 tbr, 30k tbn, 59.94 tbc
    Stream #0.1(eng): Audio: aac, 44100 Hz, stereo, s16, 62 kb/s
    Stream #0.2(fra): Audio: aac, 44100 Hz, stereo, s16, 62 kb/s
    Stream #0.3(deu): Audio: aac, 44100 Hz, stereo, s16, 62 kb/s
    Stream #0.4(spl): Audio: aac, 44100 Hz, stereo, s16, 62 kb/s
    Stream #0.5(ita): Audio: aac, 44100 Hz, stereo, s16, 62 kb/s
    Stream #0.6(pbr): Audio: aac, 44100 Hz, stereo, s16, 62 kb/s
Output #0, mp4, to 'new_multitrack.mp4':
  Metadata:
    encoder         : Lavf52.64.2
    Stream #0.0(eng): Video: libx264, yuv420p, 720x404 [PAR 101:135 DAR 4:3], q=2-31, 1280 kb/s, 60k tbn, 29.97 tbc
    Stream #0.1(eng): Audio: mp2, 44100 Hz, stereo, s16, 64 kb/s
Stream mapping:
  Stream #0.0 -> #0.0
  Stream #0.1 -> #0.1
Press [q] to stop encoding