My wife has a 98 RAV4, with one of those keyless remote-controlled carlocking systems. Last week, she noticed that the shrill sound that accompanies the locking and unlocking process (by pressing the button on the remote) would sometimes not be heard, although there would be nothing wrong with the locking/unlocking mechanism.

Yesterday, the shrill sound disappeared altogether. The locks still respond to the remote. It's just that the sound is no longer there. She sort of wishes to restore the sound. After all, it does help in indicating whether the mechanism has indeed responded to the remote.

What could have gone wrong with the sound? Any way I could fix this? Hope somebody can help.