Normal earplugs definitely will kill certain frequencies and change what you are hearing drastically, however using Etymotic style earplugs are a lot better, as

Normal earplugs definitely will kill certain frequencies and change what you are hearing drastically, however using Etymotic style earplugs are a lot better, as they negate the high pressure waves, but still let a broad range of frequencies through - and they still allow normal talking levels when away from the music so you don't have to keep taking them in and out.