—We were only thinking about it, Stephen said.
—All the talents, Myles Crawford said. Law, the classics…
—The turf, Lenehan put in.
—Literature, the press.
—If Bloom were here, the professor said. The gentle art of advertisement.
—And Madam Bloom, Mr O’Madden Burke added. The vocal muse. Dublin’s prime favourite.
Lenehan gave a loud cough.
—Ahem! he said very softly. O, for a fresh of breath air! I caught a cold in the park. The gate was open.
Omnium Gatherum means a collection of miscellaneous people or things. I can only assume that Joyce is pointing out that at this point he filled the room with so many random people all with very little description thus making it hard to keep them all straight at first reading. This entire episode has been difficult to draw because of the number of people standing in the room.
I assume Lenehan cleared his throat when Mr O’Madden Burke brought up Molly because he knows something is going on with her. It seems everyone seems Molly is unfortunate to be married to Bloom.