I just realised I was way too brief on my post, so let me elaborate:
If a coworker/friend/roommate has your password they are definitely pranking you:
1. Check the number 9 and 10 on this site.
2. Another example of your mac saying things "out of the blue".
3. Tip number 4 "But the most enjoyable use for say is rather more sinister: if you have ssh access to a friend or coworker’s Mac, you can silently log into their machine and haunt them through the command line. Give ‘em a Siri-ous surprise."
So, it is more likely a prank being played on you. Change your user's password and NEVER, EVER give anyone your password.
I have never given out my password and I work alone from home so nobody I know is playing a prank on me. Safari had a webpage open so could it have been just a sound file played on that site?