Haha... Great point by both of you.
Sound "inside" a vessel, has air & so is heard... so an explosion for example on the hull would be heard. Lets just say that blaster fire is represented by what the person shooting it is hearing as it leaves the gun & airwaves transfer the sound into the cab. The sound the TIE fighters make are the sound that the driver of that vessel hearz... etc...
Since Star Wars is just a cinimatic representation of true historical events. (from a long time ago)... & as it's only a representation... they added the sound to these historical biographies for the sake of the audience... as heard from inside each vessel.