New apple keyboard Fn keys don't work

Did you open System Preferences +Keyboard & Mouse+ pane Keyboard tab. There should be a setting there to enable (use the special key functions) or disable (use the special keys as regular F-keys) that feature. Make sure it is set properly.

When I open two Finder windows to

<startup drive>/Library/Preferences/
and
<user home folder>/Library/Preferences/

and show as list, sorted by +Date Modified+, and then make a change to that setting (in System Preferences +Keyboard & Mouse+ pane Keyboard tab), the preferences file that gets updated is

<user home folder>/Library/Preferences/com.apple.universalaccess.plist

So I would try moving that files to the Desktop so that a new one gets created. Then try resetting that preferences in System Preferences +Keyboard & Mouse+ pane Keyboard tab.

Also (if that does not work), you should make sure this SizzlingKeys did not install any extensions into your System. If anything is there, they would most likely be at

<startup drive>/System/Library/Extensions/

and (hopefully) have a file name that includes "SizzlingKeys" as part of the name.

I would try disconnecting (leaving off for a few minutes) and reconnecting the keyboard.

If that does not work, try creating a new admin user account it System Preferences Accounts pane. Log out and log in to the new user account. Does it work or not work with the new user?

If it does work, there is probably some other preference file in your usual account that is interfering.

If it does not work, the problem is with the overall system, not something in your user account.

I've used SizzlingKeys in the past. It was novel for a few months but conflicted with keyboard commands I was used to in other applications. The iTunes library on that computer is no longer used. However, I see that SizzlingKeys 1.7.2 is still installed. Preferences depict only two SizzlingKey options enabled: Play/Pause and Almost Mute. The rest have their checkboxes clicked off. Here, at least, there is no conflict with the Apple keyboard functions on that G4 computer running Panther.

It seems awfully odd that a removed application could still be causing a conflict. If SizzlingKeys is the culprit and nothing else worked, I (perhaps not you) would be inclined to reinstall it. Whatever iTunes actions are mapped to F1 through F4 and F7 though F12 can be disabled or changed within the SizzlingKeys Preference Pane. There is even an option to disable SizzlingKeys altogether. Should you then decide to remove it again, do so after disabling everything in the Preference Pane. Between this and the Keyboard Shortcuts button labeled "Restore Defaults" (Keyboard & Mouse Preference) the problem should be resolved.

(FYI: If you were using a notebook computer, there would be another option to toggle that requires the extra Fn key to the right of the Control key to be pressed for access to special Apple functions. But since you posted this in the G5 forum, this apparently doesn't apply.)

Do those keys work with other Apple media playing software, such as QuickTime Player and DVD Player?