[FFmpeg-user] m4a won't play on Alexa

[sean darcy]

I've got a bunch of sonatas I've uploaded to Amazon Music. They were 
originally opus files, but Amazon requires mp3 or m4a . ( Why ? Got me.)

So I re-encoded the opus files:

ffmpeg -i sonata.opus -vn -c:a aac -b:a 128k sonata.m4a

The m4a files play just fine on my laptop, even in the Amazon Music 
windows app, but not in Alexa.

"Sorry, I am having trouble playing the music."

In the Alexa forum I found the following helpful advice:

Make sure you are not sending a content type header of video/mpg4 which 
would be wrong if this is an audio-only stream. Also, it could be that 
the encoding is incorrect, for example the moov element is not present 
in the stream, etc.

Any suggestions on how to encode that's Alexa proof ?

sean