If you can see the issue on another layout / file / mac, report it as bug.
when I import a .m4a-file in a container field, I have no problems to see the start/stop symbol but no progress bar. Strange enough - using a WebViewer field I get stuck when refering to the respective local path.
However if your problem is related to my post under „Report a product issue“ … Audio container progress-bar missung FMP15 = https://community.filemaker.com/message/567928#567928
then that workaround may help.
What you need to add is the suffix .mp4 to your suffix .m4a. Example: with the function GetContainerAttribute you get the local path showing at the end for example … /mymusic.m4a. Here you only would have to write an additional script line to extend this entry to receive as path reference the text ending with … /mymusic.m4a.mp4. = with the function "set field" and then calculatetd field … „the original local path“ & ".mp4“
The problem vanish if I restart Filemaker. It comes back after a while, but as mentioned a restart is enough to fix it. I don't have time to bother more with it at the moment.