Applicatons-Settings-voice input and output-text to speech-driving mode (enable or disable)
If you disable driving mode, you will not have any more announcements, if you want to have it enabled, go to the driving mode settings and select the items that you would like the phone to announce. The unlock screen announcement is at the bottom of the settings.
Note: These steps are for a stock, unmodded ROM.
Samsung Galaxy S2 Skyrocket
Running: Embryo 6.11
HTC Tilt2 - Simplicity
HTC Pure - Energy
HTC Tilt - Stock, themed