If I put a bunch of time and effort into creating non-text content, namely audio and video, how will Google grasp how to index it? Is there a way to "prep" audio and video so it gets noticed like text gets noticed?
Use meta data like description, title to tell Google about your audio/video.
Google is planning to "listen" to your audio/video too for analyzing but it's in a nascent stage right now.
<meta name="title" content="Best Video for startups" />
<meta name="description" content="Check this video on startups by Kenneth" />
Put video on YouTube and if it's relevant it'll show up. As you put titles and descriptions on the video, it'll get indexed and if there isn't too much competition it should have a chance of showing up in the top half of the results (where Google tends to put YouTube results). For example, do a Google search for "how to tie a tie". You'll see that despite not mentioning 'video' in the search, a YouTube video for it appears (for me) in third position. So whilst there isn't anything particularly clever in how Google does this, from a marketing point of view it works.
So yes, I believe it is worth spending time on video content (not as sure about audio).
Indexing media, beside text and similar, is hard and currently it relies on metadata that it can gather.
For instance, a MP3 that has the artist, song name, etc. or a youtube vídeo that has tags.
I believe it will take a while until non-text media is indexed as easily and accurate as text is nowadays.
Text is still king and it will rule for a while...