Dancing to "Tarantara" from Pirates of Penzance and preparing for his first musical chairs competition.
For those unfamiliar with the operetta, the police are singing, "We go. We go. Yes, forward on the foe" while wandering around in circles. The Major General sings his annoyance that they don't actually go, during which they pause. This is Hyrum's faithful reenanctment. "Tarantara" is their version of the "trumpet's marshal sound" which they recite to compensate for an innate lack of courage.
A little later, the pirates are making their climactic attack on the Major General's castle, singing in their loudest possible voice that their appreach is silent and stealthy, "With Cat-like Tread." During the movie version (thank you, Youtube!) they take a step, swing their swords for 3 counts, then take another step on the downbeat. This is Hyrum's reenactment.
UPDATE: Right this second, on the way back from a diaper change and Daddy singing Tarantara, he opened his mouth and spake, saying "I want Tarantara song, please."