Appropriate Use: When to Use Each Type
I use both of these annotation types more than I thought I would when they were first introduced, back in Acrobat 4 (or so).
I tend to use Sound attachments mostly when I’m making a comment that is general rather than specific, commenting perhaps on the philosophy of the document rather than a specific strength or weakness. It seems to be easier to speak my tiny mind than to write out a vague concern or comment in a sticky note. Also, an audio annotation can be much more effective than a sticky note; a spoken comment can get people’s attention much more than a written note (especially if it’s the screeching chimpanzee sound).
I use File Attachment annotations only when it’s very important that the PDF file and its accompanying support document (a spreadsheet file with the financial data, perhaps) not be separated. Sending the files as individual email attachments (or even zipped together) always invites one of them to be misplaced by the recipient. (Note that Acrobat can also bundle together multiple, arbitrary files as a PDF portfolio, but that has a lot of UI overhead that is very often unnecessary; File Attachment annotations are less fancy, but they’re smaller and get the job done perfectly well.)
There’s a benefit to the fact that Sound and File Attachment annotations are uncommonly used. Because of their rarity, they get people’s attention and impress the recipient with the breadth of your PDF skill and knowledge. So, give them a try!