why is my snow leopard mac speaking 'welcome to rmit' when i join a network?
Something is running a process when you connect to your home network. You said "Locomatic is now disabled" - have you actually uninstalled it?  Probably worth a try.  Here's the Manual.

You might also lay a trap for say by temporarily renaming /usr/bin/say to sayx, writing a wrapper script in /usr/bin called say that calls sayx and passes its arguments to it, but which doesn't exit when sayx returns.  The next time your system speaks up, you can look at the process list with ps -f or Activity Monitor and trace through the Parent Process IDs to find out who launched what, leading to the announcement.  In fact, I'd probably do this first to make sure I know about everything that might need cleaning up.