It's just an onomatopoeia that mimics the sound of the drums, it's hard to transcribe!. If I'm not mistaken, in the script published by Samuel French they just give the stage direction "(Velma imitates drums)".
There are no words written into the conductor's score either but the previous poster is correct that Chita is just mimicking the sound of the drums. It's always sounded to me like she says, "boomdabash boomdabash boomdabash that's the drums".