For some reason your system volume is down. Download audio manager from the market. It's free, and use it to increase your system volume. And as long as you have unlock sounds ticked in your settings it'll work.
Sent from my HTC Desire using the miuiandroid.com forum app
I had this problem last week. I've always had lock sounds enabled then one afternoon after listening to music through my headphones after unplugging the headphones my lock sounds disappeared. A reboot didn't fix it either. I then noticed the next day that the lock sounds would play fine when the headphones were plugged in but not when they weren't. All other notification and system sounds were fine. I managed to fix this by putting the phone into silent mode plugging the headphones in taking it off silent and then unplugging the headphones. Lock sounds work fine now.
Sent from my HTC Desire using the miui-dev.com Forums App