The changeover to Pulseaudio in Hardy has resulted in a bug with Flash. In particular, using the libflashsupport package can result in a lot of crashes, so it has been made into an optional package when the flashplugin-nonfree is installed.

Unfortuantely, a side effect of making libflashsupport an optional package is the problem you're describing; Flash can commandeer the sound card and not let other apps use it.

A fix for this being worked on. The gory details are in bug #192888.