Fun to embed m4a files that could just do this all by themselves?-)

I think the most disappointing thing about these is that you cannot quite record over them (over zip files) in real time, unless you arrange the particular m4a files to be wavs with e.g. PCM or A-law, or I suppose some more sophisticated but constant bitrate codec, and then also arrange that file to be uncompressed in the zip.. ? I doubt there are libraries to help make that happen?

At least if they were just plain mpeg4 containers, the reduced amount of indirection would make the problem a bit easier and could probably be manipulated with GPAC/MP4Box almost just as easily as zip files.