There are multiple speech plugins for different administration programs. Barring the built-in half-life voices, when a player connects you would have to make sure each client downloads a specific audio file that you plan to trigger. The custom content needs to be loaded into a .res file for each map on your server. Below is a little program my brother whipped up to automate this task. It was designed with Counter-Strike in mind but it will work for any half-life mod.
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<a href='http://www.vadakill.com/files/ResMagic/' target='_blank'>http://www.vadakill.com/files/ResMagic/</a>