Using the My Cloud app on an iPhone, I can access my MC NAS via my home network, or any remote hotspot (e.g. xfinitywifi) and play any music files, incl. m4a files. (I do not use the mycloud.com program on the PC.)
The iOS MC mobile app cannot play from playlists, although I have lots of them. Playlists can only be played from UPnP sources, not Samba (shared) sources and the MC app only accesses and plays from the NAS as a samba/shared source, and not a DLNA/UPnP.
The MC NAS can serve media files via DLNA (i.e. UPnP) if a UPnP app such as BubbleUPnP, or Avia is used on an Android device. For example, I have both BubbleUPnP and Avia appsinstalled on my Kindle tablets so I CAN play my playlists on the Kindles (and can beam the music playing elsewhere via Bluetooth if I want to; say to a BT speaker).
I also can play the playlists (stored on NAS) on home devices that can access via DLNA, such as a WDTV that can access the NAS as either a media server or network share. I just use the WDTV’s media server access method (network shares access can also play the music files, but cannot access the playlists – only media server access can do this.
Hopefully, this explains why playlists can be played or not played. Playlists need to be played via DLNA/UPnP only.
BTW, I have requested to WD that the MC iOS app be designed to also access media via DLNA as well (so we can play from Playlists) but so far, the feature has not shown up in the MC app. My thinking is, if it can happen with the WDTV, why can’t it be done with My Cloud app!