Did you try to send the same M4A file to Pioneer using different control point (e.g. Windows Media Player on Windows, BubbleUPnP on Android) to be sure that Pioneer always refuses to play the file or whether there is a way how to send the file for Pioneer to play it?

As for the codec pack, just installing won't do the trick. You need to set auto-convert rules for the M4A file to be converted to MP3.
Set it in Options > Media Sharing > MediaMonkey Library > Auto-conversion tab :
https://www.dropbox.com/s/8xedsq7ei5l27zp/Screenshot%202018-12-13%2012.50.56.png?dl=0

Hi,
Thank you for patience, can you please supply us with log file as described at https://www.mediamonkey.com/forum/viewtopic.php?f=1&t=69 in ticket that you opened?

EDIT: Also one sample file could be useful. You can simple add download link in Ticket.

Hi Lowlander, I've checked that and m4a is listed as a supported format. Also I have loaded the trial mediamonkey codec but that has not fixed it.

Again any ideas gratefully received.

It would be up to the Pioneer to be able to play the format. If it does, make sure it's enabled under Supported Formats in the Media Sharing settings. Otherwise you'll need to convert from it to a format understood by the Pioneer.

Ahh I just read this post - https://www.mediamonkey.com/forum/viewtopic.php?f=20&t=61382&p=395344&hilit=decode#p395344
Look like I need the MM Codec Pack - I'll give that a try.

Hi Peke (hope I got that right), thanks. I tried QT-Lite, then tried installing iTunes. Then half a dozen reboots. Current situation is that I can play m4a to the local MediaMonkey player but I cant cast them to my Pioneer network player over UPnP/DLNA. It has worked in the past with exact same files. I get the message "MediaMonkey is unable to decode this file" and it prompts me to find missing Codec's. Any thoughts?

Hi,
Have you tried http://www.free-codecs.com/download/qt_lite.htm ?

Also As QuckTime is not distributed separately, but is included in iTunes installation you can also install iTunes to get m4a support.

Have you tried MMW codec Pack also?

Hi - I have MMW gold recently re-installed, It is refusing to play m4a files saying it does not have the codec. I have tried installing various codecs but the problem persists. Running Win10. Incidentally Groove plays m4a fine, not sure if that uses codecs or not. Any advice gratefully received.