Keyless entry problem on my 2000 Lexus Es300...?

I have the keyless entry on my car, everytime that I would lock or unlock the car it would make a beeping sound, sort of an announcement that car was just locked or unlocked. But now I hit the button on my key to lock it and unlock it and it still locks and unlocks but it just doesn't make the beeping confirmation sound. What could be the problem? Thank you in advance.