MSO? Not sure what that is straight off.
What I want is to have a piece of text underlined, and when I click/tap on it, a pop-up window appears with a definition. Some of the definitions have illustrations.
Alternatively, when I click/tap on the text, it takes me to another page (glossary), but I would need to be able to return to the page I was reading after reading the glossary item. None of this seems possible with current ID interactive features, as far as I can make out.
BTW, this is a dance instruction manual, which did very well in print form (sold nearly 60,000). "Definitions" are very needed in this genre.
I have successfully set up the relevant music on each page with start-stop-pause buttons, but playing always stops when moving to the next page. Another problem yet to solve.
BobLevine means Multi State Objects: it is very easy to add invisible buttons over pieces of (underlined) text, and control a multi state object which you place on the page. With a simple button action the required MSO state with the definition and illustrations can be shown to the user. It works like a popup. In each MSO state you should add another button with an action to close the MSO (or rather, direct it to display the 'invisible' state).
As for you question related to ambient background music playing throughout all the pages of your book: this cannot be achieved in InDesign (well... there might be a trick, but it's a heavy-handed workaround, not certain to work), and requires some hand coding (and will only work in iBooks, as far as I am aware). Read page 69 of Apple's iBooksAssetGuide: https://itunesconnect.apple.com/docs/iBooksAssetGuide5.1Revision2.pdf
https://itunesconnect.apple.com/docs/iBooksAssetGuide5.1Revision2.pdf
This page explains how you add an ambient soundtrack to your ebook.