This page is for all users of Ubuntu.
In their infinite wisdom, Ubuntu decided not to include pavucontrol in their default applications even although they enabled Pulseaudio by default. The reason for this relates to the apps non compliance to the HIG, which is a laudable goal, but sadly means that there is no way for users to control pulseaudio.
Many users, it seems, resorted to padevchooser, but this is a terrible application that does not interface with Pulseaudio in the correct way and is also very misleading with regards to how to make your sound come out of the right device (or sink).
Setting a 'default' sink
This is a misnomer straight off. Pulseaudio does not really support a 'default' device in the way most people think. Pulse remembers which sink your application was last used with and restores it to that sink when it plays again. This means that the 'default' is only used the very first time an application uses sound. It should really be called a 'fallback' sink. Work is ongoing to support a more 'active' default.
How do I make App X, output to Sink Y?
Firstly, install pavucontrol and then run it. When you app is playing sound, it will appear in the Playback tab. Use the little menu there (with the down arrow on it) or right click on it and select "Move Stream". Pulse will remember this decision and will use this new sink from then on in for that application.