In OS X(10.4.11) I am unable to allocate volume to F11 and F12 how do I do this

I have an intel OSX 10.4.11 iMac and recently needed to replace the keyboard. Apple extended USB keyboard installed but function key allocations do not work as expected. Turned off standard function keys in system preferences and allocated F3 and F4 to expose and dashboard in the expose pane of system preferences but cannot discover a way to allocate the volume up and down to F11 and 12. Please advise if there is a solution. Thanks. Sherry