I went to the Evernote forums on their site (you have to sign in and choose the Help option. A search for 'iPad amr' netted one thread. Apparently they depend on the built in audio recording ability of android phones, and these usually only supply amr.
The only solutions I can think of is to use another Android note taking program that records in a more compatible format, then attach it in Evernote, or Using Open In... in Evernote to open the .amr file in another app. But I can test either option. I don't' have an Android phone anymore for the fist. I can't seem to find a sample amr file to test with the other. I suspect that OPlayer will play AMR files, but it does not say so, exactly, so both the Open In and actual playing need testing.
I suspect the Evernote does not support Open In... because I could not force it with a .wav file, and that should have been possible; but I won't be sure until I find a file attachment it does not support, and one of my other apps does; if that makes sense.
But if OPlayer (or some other app) will play the files, then you sould at least be able to mail the notes to yourself and then use Open In in Mail to copy and play them in the compatible app.