"Myth makes Echo the subject of longing and desire. Physics makes Echo the subject of distance and design. Where emotion and Reason are concerned both claims are accurate.
"And where there is no Echo there is no description of space or love. There is only silence." (Danielewski 50).
These words finish about a 5 page discussion of the physical and mythological Echo, and sums up the topic nicely. This is one of my favorite moments in the text, and succinctly goes a long way toward providing some sort of "meaning" for house of leaves.
Think of it this way: the sound/echo is language/thought and the walls which this sounding bounce back from are knowledge and the known. We use language/thought to place ourselves in this world, and we do so by bouncing our ideas/words/thought off of our knowledge base--things we know to be true. HOL builds a world where the usual knowledge base is shifting and unreliable, language is transforming, and the truths we use to place ourselves become irrelevant.
-Cole, Hannah, Roberto