Activ uses the same Windows components that IE uses, even if your user has a different browser for surfing.
As far as building your pages goes, a lot of folks use Word to generate their web pages. Word creates web page code full of extraneous XML and proprietary markup, intended for users who want to re-import the web pages back into Word again.
If you use Word, you might want to visit this first link, which provides a 'clean html' option. I would also recommend running a cleanup utility on those pages. You'll have a much smaller set of files, and thus a smaller, faster ebook.
Here are two free utilities, one from MS itself: a Word Html filter:
and TidyHTML, a nice cleanup and formatting utility:
Both Dreamweaver and FrontPage accepts Word html, and can clean them up as well.
There are also several free html editors of varying quality; Arachnophilia, 1st Page, and HtmlKit are the first to come to mind.
As far as audio goes, html allows you to play some embedded sounds; you've no doubt encountered sites that start some sound or music upon opening. Search Google for 'html embedded sound' and you'll find a lot of examples.
Sunil also has a multimedia add-on, the 'Advanced Multimedia Extension', available at the 'EBookPower' link at the bottom of this page, to make the process of adding sounds easier.