Though the Supported file types section of Settings > Community Settings > Media Settings says "we support all audio files", this unfortunately isn't the case. Even after .m4a is added to the list, m4a uploads will generate an error.
I appreciate it's probably a little niche but I'd love it if this file type could be supported, as it would allow users to upload voice notes directly from their mobile devices.
Adding here in case others feel the same!