The proliferation of high-speed Internet access and powerful computers with massive hard drives has made it possible for you to deliver more of your content in audio and video format. But just because you can, that doesn't mean you should. At least, not exclusively.
Granted, many people are visual or auditory learners, and a picture can definitely paint a thousand words, but A/V content seems to work best as an accompaniment to written material, not as a replacement for it.
The main problem with A/V content is that it is serial and linear. You can't "skim" it. You can't "speed read" it. You are stuck accepting the information at the pace it was originally recorded. Sure, you can fast forward or skip around, but that process is slow, cumbersome, and leaves you with no idea of what you might have skipped.
How many audio programs have you downloaded with the intent to "listen to it one of these days," but you never have? Conversely, how many times have you done an Internet search and quickly scanned dozens of pages to locate the information you need?
The book industry is seeing a rapid transition to electronic media with supplementary content in the form of audio, video, and Web page links. But that doesn't mean books are going away. It just means that digital books will provide extra resources that improve the learning or entertainment experience for readers who enjoy and benefit from the additional materials.
The key is to make sure you don't segregate any important part of your message into these alternative formats. When considering what content to provide in audio or video format, think "supplementary" or "alternative," not "essential." Your audience, or more to the point, your readers, will appreciate it.