While having a look at the linked zip file you pointed to, I found this
in the included word document:
Copyright Notification
No part may be reproduced except as authorized by written permission.
The copyright and the foregoing restriction extend to reproduction in all media.
Sorry, this means we cannot include it into our ffmpeg tree.
I suggest you work together with ffmpeg upstream to get a free
implementation of AMR into ffmpeg.
** Changed in: ffmpeg (Ubuntu)
Importance: Undecided => Wishlist
Status: Unconfirmed => Rejected
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ffmpeg does not have AMR audio support
https://launchpad.net/bugs/93849
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