Continuity mistake: At the beginning, when the Expendables and Trench are leaving the interrogation room, we see Jet Li along with his main weapon leaving with them, but in the next shot, he picks up his weapon and leaves the kitchen.
Continuity mistake: Just after Billy dies, Barney says "let's take care of him" and Toll Road puts his cap on. In the very next shot as they walk away Toll Road's cap has disappeared.
Answer: The words of their chant are "Beware, beware, walk with care. Careful what you do. Or Mumbo Jumbo's gonna hoo-doo you. Mumbo Jumbo's gonna hoo-doo you. Boomlay, boomlay, boomlay, boom!" This comes directly from a poem titled "The Congo" by poet Nicholas Vachel Lindsay ("The Congo and Other Poems"), specifically the lines at the end of part I.
Super Grover