Edit: After half day, my wife’s Mac started having the same problem. I have then applied the solution that I found in the Apple
Siri Dictation Now Available for iPhone 4 –
It was found out that Siri did not require the new A5 proccessors or any other new technology a few weeks back. So since that news hackers have been working hard on porting siri over to other devices. We have just found out it is now possible to run Siri diction on a iPhone 4 via jailbreak. A tweak has been released called Siri0us and is describerd as Siri dictation for your iOS 5 devices. No iPhone 4S keys/files required. To get Siri dictation on your iPhone 4 download Cydia (Jailbreak your device) then simply add http://apt.if0rce.com/ to your sources and search for “Siri0us.
Update : Maybe have to try an download a few times