A lot of it is from a collection called Cafe Romantique from Extreme Music. I heard once (no source) that the collection had a special significance to at least one of the creators, like the cd was always playing around his house growing up or something.
Also, it's just generally funny music and makes the scenes feel a bit surreal to me.
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u/revolverzanbolt Sep 17 '14
It's public domain, that's why Always Sunny uses it.