CORRECTION: No one really thinks that eventually the Cot-Caught merger will spread to all of the US. There are some dialects where the vowels have drifted really far apart rather than merging together.
If you're curious how they sound unmerged, check this video out: https://www.youtube.com/watch?v=RB3NHCJDq7M
Next video, the one on Grimm's Law:
http://www.youtube.com/watch?v=VnjfHu9eJLM
Intro Song: https://www.youtube.com/watch?v=BR7IOOLAiRo
Outro Song: https://www.youtube.com/watch?v=yFYSYGLx-RM
