mute key to go into standby was a glitch that was removed in newer OS releases.
there is no fix since it has been fixed, unfortunatly some people liked the glitch.
you can use a password to lock the device and use a hot key "K" to lock it with stinsondog tips to enable keyboard shortcuts.
Blackberry Tips & Tricks from the Mods
according to this KB it was intended only for track ball devices.
KB13025-How to turn on Standby mode using the Mute key
When the Mute key is pressed, the BlackBerry smartphone does not enter Standby mode
KB22586-When the Mute key is pressed, the BlackBerry smartphone does not enter Standby mode
take note on products affected on right side.