just to clarify, iNaturalist does accept m4a files. you can for sure upload these in the web upload screen although i’m not sure if the iOS apps accept them. however…
it does seem like iNat potentially makes the quality of m4a files worse upon upload: https://forum.inaturalist.org/t/issues-with-quality-of-sound-recordings/52178/22, and as far as i know, that issue has not yet been resolved.