Finally, we get the point of Mulligan's joke. He's using the echo of the surrounding mountains* to add to his travesty of the mass, as we've said several different ways and times already.
In responding to this picture, I started to think about ways in which Mulligan's performance is a mini-model of the book as a whole... but I don't really buy that.
We haven't talked much about textual issues yet, but it's interesting here that the Rosenbach manuscript does not have the sentence on which this image is based: ["Two strong shrill whistles answered through the calm."] I have no intention of subjecting you to the full textual history--there's plenty of that on the web already. Suffice it to say that the Rosenbach Manuscript, which is part of the collection of the Rosenbach Museum & Library in Philadelphia, is an early handwritten draft of Ulysses.
Why add this sentence? The skill of 'placing' the echo or the whistling mailboat says something about Mulligan's cleverness and awareness of his surroundings. It's likely also something that Joyce saw happen while he was writing, so he decided to stick it into the book--the man liked to add new material to the book whenever he got a chance. This is a small, but useful example.
*Many readers believe the whistle is the call of a boat departing Kingstown harbor, perhaps the mailboat at 5:83 (Gabler). This also seems logical to me, and probably would better explain why the returning whistles are "strong" and "shrill." But if we go this way, we have to explain how Mulligan knows when the whistle will sound. I note above that the "strong shrill whistles" are not in the Rosenbach Manuscript. Neither is the mailboat reference. So that makes it likely that the whistles are the boat... oops!