Exactly what happened to Orcrist at Thorin's funeral isn't completely clear. The Hobbit tells us that the sword was placed by Thranduil 'upon his tomb', and that it 'gleamed ever in the dark if foes approached' (The Hobbit 18, The Return Journey). These quotes seem to suggest that Orcrist wasn't actually buried with Thorin, but instead just laid on his tomb.
In The Lord of the Rings, there's a suggestion that this may have changed. We are told there that Orcrist '...lay now upon the breast of Thorin...' (The Fellowship of the Ring II 3, The Ring Goes South). This seems to suggest that the sword was actually within Thorin's tomb, and not just placed on it, though this text is admittedly ambiguous.