GFork

What it can do:
* play a sound when you have a message in your inbox
* play a sound when someone you follow reblogged your post
* can disable sound notifications if you're currently viewing the page
* load new posts from your dashboard and put them onto the top

How to temporarily disable notifications: there will be a switch next to "Account" menu in the top-right corner. That switch controls notification mode (always/only when page is inactive/never).

Known issues:
* post controls are broken for autoloaded posts
* reblogs by non-followed blogs are only detected if your dashboard is updated regularly
* script depends on Tumblr builtin method of delivering update notifications to find if something has changed
* script might conflict with scripts that change post height/count, which will result in page being scrolled incorrectly on updates