How can I tell what "Audio" and "Other" contains in iTunes's iOS device capacity meter?
The other part of the graph, as far as I know, consists of caches, cookies, history, logs, ... (mostly from safari). Also, things like contacts, calender events, ... are in this part, too. I don't think any app document (like Evernote's or Instapaper's) goes to this part. It's all in the app part. 

So, for example, it can be cleared up a good amount by going to safari settings and clearing all caches, history, ... You'll be amazed at how much space they've claimed!

About the audio, it is really strange that with no music and some audiobooks, it have taken up so much space. I don't know any other stuff other than that to be included in that part.

As a side note, sometimes doing a resync in iTunes will change the graph. Sometimes even the other part shrinks and goes to apps. So maybe you can try and see what happens.

Hope it helps.