I stripped one of these m4a files into a raw audio file, and then re-encoded it to .m4a to strip any unusual metadata, images, etc... created a dummy .rss feed and it was able to play on podbird... however, the process of stripping down to PCM audio revealed the nature of the unexpected metadata.

It appears to contain Metadata including Chapter information for each track in the mix, as well as subtitles, and mjpeg video streams..  it's made with garageband, so perhaps if someone has that software they could see what data/images are actually there.

That said, it plays fine in every other player I've ever seen!

  Metadata:
    major_brand     : M4A 
    minor_version   : 1
    compatible_brands: M4A mp42isom
    creation_time   : 2015-06-29 17:05:42
    composer        : Above & Beyond
    title           : ABGT137
    artist          : Above & Beyond
    album           : Group Therapy
    encoder         : GarageBand 6.0.5
  Duration: 02:00:00.16, start: 0.000000, bitrate: 194 kb/s
    Chapter #0.0: start 0.000000, end 357.100000
    Metadata:
      title           : 1. Jason Ross ‘Ravenna’ (Anjunabeats)
    Chapter #0.1: start 357.100000, end 627.091667
    Metadata:
      title           : 2. Sound Quelle & Max Meyer ‘Monolith’ (Statement)
    Chapter #0.2: start 627.091667, end 880.450000
    Metadata:
      title           : 3. BT & Super8 & Tab ‘Aika’ (Anjunabeats)
    Chapter #0.3: start 880.450000, end 1201.850000
    Metadata:
      title           : 4. ilan Bluestone ‘Bonsai’ (Anjunabeats) - Record Of The Week
    Chapter #0.4: start 1201.850000, end 1460.441667
    Metadata:
      title           : 5. Sub Question ’Semiconductor’ (Elliptical Sun)
    Chapter #0.5: start 1460.441667, end 1684.766667
    Metadata:
      title           : 6. Intrinity feat. Erika Faries ‘Don’t Look Back’ [Proglifter Dub Remix] (PHW)
    Chapter #0.6: start 1684.766667, end 1971.325000
    Metadata:
      title           : 7. Otto Knows ‘Next To Me’ [Grum Remix] (Big Beat)
    Chapter #0.7: start 1971.325000, end 2181.333333
    Metadata:
      title           : 8. Pierce Fulton ‘In Reality’ (White)
    Chapter #0.8: start 2181.333333, end 2515.751667
    Metadata:
      title           : 9. Third Party feat. Daniel Gidlund ‘Collide’ (Release)
    Chapter #0.9: start 2515.751667, end 2748.236667
    Metadata:
      title           : 10. Cuebrick ‘Demon’ [Kago Pengchi Remix] (Enhanced)
    Chapter #0.10: start 2748.236667, end 3033.583333
    Metadata:
      title           : 11. Las Salinas ‘London’ (Vandit)
    Chapter #0.11: start 3033.583333, end 3338.591667
    Metadata:
      title           : 12. Cosmic Gate & Mike Schmid ‘No One Can Touch You Now’ [Rodg Remix] (Wake Your Mind)
    Chapter #0.12: start 3338.591667, end 3593.911667
    Metadata:
      title           : 13. Jean-Michel Jarre & M83 ‘Glory’ [Steve Angello Remix] (White)
    Chapter #0.13: start 3593.911667, end 3882.693333
    Metadata:
      title           : 14. Above & Beyond feat. Zoë Johnston ‘Peace Of Mind’ [Arty Remix] (Anjunabeats) - Push The Button
    Chapter #0.14: start 3882.693333, end 4140.643333
    Metadata:
      title           : 15. Max Freegrant & Digital Sketch ‘Counting Down The Days’ (Freegrant)
    Chapter #0.15: start 4140.643333, end 4450.328333
    Metadata:
      title           : 16. Mark Knight ’Second Story’ (Suara)
    Chapter #0.16: start 4450.328333, end 4767.750000
    Metadata:
      title           : 17. Dusky ‘Skin Deep’ (17 Steps)
    Chapter #0.17: start 4767.750000, end 5100.041667
    Metadata:
      title           : 18. Ashworth ‘Cork’ (Needwant)
    Chapter #0.18: start 5100.041667, end 5365.936667
    Metadata:
      title           : 19. M83 ‘Midnight City’ [Eric Prydz Private Remix] (Mute) - Flashback
    Chapter #0.19: start 5365.936667, end 5637.111667
    Metadata:
      title           : 20. Erkka ‘ID’ (White) - Guest Mix
    Chapter #0.20: start 5637.111667, end 6011.361667
    Metadata:
      title           : 21. Erkka ‘Talk To The Hand’ (Anjunadeep) - Guest Mix
    Chapter #0.21: start 6011.361667, end 6319.386667
    Metadata:
      title           : 22. Erkka ‘Plastic Flamingos’ (White) - Guest Mix
    Chapter #0.22: start 6319.386667, end 6665.016667
    Metadata:
      title           : 23. Erkka ‘ID’ (White) - Guest Mix
    Chapter #0.23: start 6665.016667, end 7200.158333
    Metadata:
      title           : 24. Erkka ‘Museme’ (Anjunadeep) - Guest Mix
    Stream #0.0(eng): Subtitle: tx3g / 0x67337874, 0 kb/s
    Metadata:
      creation_time   : 2015-06-29 17:05:42
    Stream #0.1(eng): Audio: aac, 44100 Hz, stereo, fltp, 191 kb/s (default)
    Metadata:
      creation_time   : 2015-06-29 17:05:42
    Stream #0.2(eng): Video: mjpeg, yuvj444p, 300x300 [PAR 72:72 DAR 1:1], 0 kb/s, 600 tbn (default)
    Metadata:
      creation_time   : 2015-06-29 17:05:42
    Stream #0.3: Video: mjpeg, yuvj420p, 300x300 [PAR 72:72 DAR 1:1], 90k tbn