You can't permanently change things on pages using Firebug. Furthermore the game you link to is based on Flash, to which Firebug itself doesn't have access to. There's an extension for it called
FlashFirebug, which lets you inspect elements inside the Flash. Though I assume it doesn't do what you want.
the client authors have no immediate plans to change their product as far as i know.
If you didn't already, you should contact them and ask if they implement an option to change the sounds. Unfortunately I don't see another solution (except just turning off the sound, of course).
Sebastian