title_body: Is it advisable to add sound excerpts to an ebook? As far as I know, many ebook readers may also play music. Does it make sense to create an ebook which contains (internal) link to audio snippets? Does the ePub standard allow it?
  
  (snippets would be copyright-free; moreover they should be internal links, so that the ebook is self-contained)
  
  [EDIT] What I am interested in is to have links to music excerpts, not a soundtrack going on while the book is read. Just to clarify it: think at an ebook about music theory. I may talk about Aeolian cadences, explain what Aeolian mode is, add a picture of an Aeolian scale... and a link which if clicked on plays an Aeolian scale.
upvoted_answer: It's great to hear about a project where the audio would actually add substantively to the content of the book--that's too often not the case. In your case, I'd say yes, it does definitely make sense to add the audio, as it is directly relevant and helpful to understanding the subject. It is also perfectly allowable in the epub 3.0 specification, though support is optional rather than mandatory--not all reading systems will have support, but there is a standardized method for those that do support it.
  
  Outside of the epub world, things are a little less rosy: Amazon sells files with audio built in, but only supports multimedia on their iOS apps. In addition, last I checked they weren't accepting multimedia-enabled files through KDP (their self-publishing program), even though the instructions for making such files are clearly available in the Kindle Publishing Guidelines. In order to sell a multimedia file through Amazon, you'll need to be able to talk directly to Amazon--if you're a mid size or larger publisher, you probably have a contact you can speak with about that.