I work for a library. We have two types of PC's in our organization, Dell Optiplex and HP 6005 Pro, both are ultra-small form factor, both experience this same problem in IE9 with different sound drivers.
When listening or watching anything online, be it random streaming video, youtube, hulu, etc. While watching, if the user unplugs headphones and then plugs them back in, the sound will no longer work until IE9 is totally closed and reopenned. Following this, the sound returns and plays fine.
The same behaviour does not happen in other web browsers. The affected PC's are running Windows 7 SP1 32-bit and are up-to-date on windows updates.