We have WB AMR content that works on the Nokia 3300. The same exact content will not work on the 3100 (plays but is corrupted), although it will accept the files and rename them to .awb correctly. While the Device Matrix does not say that the phone even does AMR (only mentions Midi), the accept headers mention audio/amr and audio/amr-wb.
I've tried reencoding the content at all the different bit-rates that the amr converter supports.
So, what's different about the 3100 (and other Nokias for that matter) relative to to the 3300?