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The theme song to Psych, I Know, You Know, is performed by series creator Steve Franks and his band, the Friendly Indians. WIKI. You can download the MP3 from the link I am providing. These lyrics were all I could find off of someone's blog.
The soundtrack also features two original songs performed by Scottish Gaelic singer Julie Fowlis and written by Alex Mandel, and one original song performed by Birdy and Mumford & Sons.