Everyone likes funny videos and music.

Tumblr is a massive creative community and social network. What
sirkibble2 said is true in the sense that there's more image posts than video/audio, but that's mostly because pics are fun and easy to make & share.
There are a ton of fan communities on the site, making it a goldmine for viral audience growth for the right content, and the reblogging feature means that even if your profile is new and unknown, all it takes is someone bigger than you sharing your work to make something happen.
The main thing I like with Tumblr is that users tend to tailor who they follow around certain interests or topics, meaning it's easier to target specific interests/tags and reach a large portion of the fan community on that site with a little luck and reblog help.