I can reproduce on iOS 8, but it looks like a browser crash, not a problem with the C2 engine. Also Windows Media Player says the resulting .m4a file is invalid as well, which probably means the encoder failed to produce a valid .m4a file. Unfortunately we don't control the .m4a encoder and due to the patent restrictions on the format it's not easy to switch to another one. So I'm afraid we have to close this as won't fix. You may be able to work around it by using a separate tool to encode the .m4a from the .wav file, then directly import the .m4a in to C2 so it doesn't convert it itself.