Have you successfully streamed .m4a files in a Player Card in the past? Someone with more knowledge of Player Cards can confirm, but the reference section in the docs states that only video and audio in .mp4 containers are supported for the twitter:player:stream value.
I can confirm as well that the twitter:player URL is opened on mobile instead playing the streaming media file, which I believe is the behavior if either (1) twitter:player:stream and twitter:player:stream:content_type are not defined OR (2) the defined stream content is not supported.