"Research has been completed/We've finished research" (same for production)
"Our labs are ready for new projects/One of our labs is ready for new projects" (when player have not set a new research for some time, like a reminder)
"UFO detected"
"UFO crashed"
"Our base is under attack!" (classic
"Alien operation detected"
"New message/New mail" (reminder to check your mail)
"Nation is pleased/not pleased (displeased)" (when relationship with any country changes)
etc.
Of course, some events happen really often (ufo detected), but i guess it is possible to set some timeout or "wait period".
What do we need:
A person with clear voice and some acting skills, plus a good microphone (or heavy editing).