One key feature missing is the possibility to add a recording object (with bookmarks) to a note sheet.
If this is held back by licensing issue may I suggest you implement the core feature and then add it as in-app purchase to select a suitable codec.
You could also have a default setting using Ogg Vorbis (completely patent free and free to use) codec together with some of the free optimized speech only codecs which result in much smaller media files but is perfectly fine for voice recording. This way you do not risk getting into having purchase a full license as I suspect you would be forced to if you included, say, MPEG Layer 3 coding (a.k.a mp3).
This is the one feature that still makes me long for One Note, apart from that you are a worthy alternative on Mac!!