Abed's "noise-cancelling" headphones are actually 3M safety earmuffs. 3Ms reduce sound by only about 30 decibels and are meant to prevent noise-induced hearing loss from long-term exposure to loud environments (construction sites, airport tarmacs, etc). They also are a comparatively affordable aid to those who might experience noise sensitivities. They are not soundproof. Abed would have been able to hear Britta's shouted spoilers pretty well.