[xubuntu-users] How to configure the sound control keys

[MR]

I'm running Xubuntu 12.04.

My wonderful wireless multimedia keyboard just bit the dust, and all I
have as a substitute at the moment is a keyboard with no multimedia
keys.

I'd like to configure some key combinations that will let me control
the volume anyway, but I can't find any place to do this.  They're not
in the keyboard settings or in the window manager settings.

Where are these configured?

Thanks.

MR