Short of sitting in a personal anechoic chamber, nothing will completely isolate you from sound, due to the physics of human hearing. You hear most of what you hear by means of your ears. Sleeeps® work by blocking sound from entering your ear canals. However, sound will still vibrate your skull, cheekbones and jaw, albeit vastly less than sound entering your ears would. This is called bone conduction.
Sleeeps® earplugs (when correctly fitted) turn the volume of the world down drastically.