An alert to who? The user? Hopefully they know when they logged in...<

It's possible, yes. But think how annoying that would be on a very active forum? There's at least one with tens of thousands of users. The voice would never finish completing the announcement.<

If nobody beats me to it, I'll write it when I'm through with version 2 of the "Buy Me a Beer" mod. I promised it first.<

Straight off, I can tell you you're going to need the active users mod to achieve this.
You could run it off the M_LASTHEREDATE field in the MEMBERS table but this would play your notification whenever a member marked all topics as read on active.asp as well.
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unless you continuously ping the server with a JS script which will slow your server down probably considerably depending on the refresh rate.<