How do I use say command creatively to prank my coworkers/friends? I'm designing a creative / funny prank for OSX via command line. By running in the background, most users would not notice it when she/he first starts using the computer.

Using say command with creepy voice (Deranged) is my best bet. Setting volume to very low during saying the sentences is even more creepy! My problem is, I don't know how to grab current current volume to set it to the same volume after saying. 

sascript -e "set Volume 2"
say "Naaame I see you" -v Deranged -r 20
sascript -r "set Volume 0"
sleep 600
sh thisscript.sh


Can I get the current volume and store it in a variable using unix shell scripts?
You could always get the volume, change the volume, say your line, and then restore the volume within the AppleScript:

#!/usr/bin/osascript
set currentVolume to output volume of (get volume settings)
set volume output volume 1
say "Hello" using "Deranged"
set volume output volume currentVolume