I got the same problem again: SMS message alert tone is not playing on my Nokia E5. No sound at all, just the blinking light. My previous suggestion, changing the home screen to another and then back, is not foolproof. So I tried deleting messages fromSMS inbox and now it's working again. Right now I have 1825 messages and SMS alert notification sound is working. It seems the threshold is about 2000 messages and it starts to "silence" itself. After deleting messages to about 1800 or under, if you still can't get SMS alert tone playing, restart the phone and then switch the home screen theme. It should work.