I do not have a whole lot of experience on multimedia rich content, but it makes sense that multimedia is not going to be of great help with your general SEO.
However, one of the recurring themes we see in discussions on SEO involve words. A client of mine recently asked me to redo their website. The existing site was well laid out, it googled well (mostly) and coded properly. The imagery was tasteful and did not obscure the text content.
However, it had one major problem.
That problem was its banner image. The image was thematic and appropriate for the site, but the title of the site was in the image as an image. That is, a search engine robot would not be able to read it, and that is a problem. The designer was trying to guarantee a specific style of font, but one has to wonder how much that font they desired cost them in terms of searchability?
It is important to make sure search engines can read your site as easily as possible. Give as many hints as you can. This is not to discourage the placement of multimedia content into your websites it is to make sure you are aware of the potential cost. If you can adequately express something in terms of HTML/CSS formatted text, then use that route, if you can not then use the means you need.
When you do place multimedia into your websites make full use of available tags and attributes to provide words the search engines can use! When putting up photos, do you fill out the alt tag? If you do not you are withholding clues from the search engine and in fact are writing code that does not conform to the HTML/CSS standards.
Use multimedia and graphical content to communicate your content not obscure it!