this may sound like a silly question, but it is serious..
example.. youve got a kilo of high explosive, C4..
the C4 is placed in the centre of a very thick block of steel and ignited. the steel is way too thick for the explosion to damage to steel. there is no way for the gasses or sound to escape... the C4 had detonated.
SInce an explosion is basically fire it will have used up all the oxygen & fule to be able to stay alive & it will die. And since sound needs an atmosphere to travel and the explosion has used up all the internal atmosphere then there will also be no sound. Thats my theory anyways.
You will be still be able to hear the explosion although muffled and there will be a hell of a lot of vibration as well. If the C4 is totally encased iit will probably deform the surround steel as well ie. compress it.
They use to test the strength of explosives by doing what is called a lead block test - see the link below -hope this is of help - http://en.wikipedia.org/wiki/Trauzl_lead_block _test
One other point - the deprivation of oxygen is likely to be irrelevant - explosion is not really fire, but an exothermic reaction that produces a lot of gas really really quickly. Most explosives include some form of self oxidation (to the extent oxygen is required to make the reaction occur). Air is relatively inneffective in delivering oxygen, and thus another oxidising agent is required.
This is why guns work even if the bullets are fully sealed.