The moment Megatron's attention (and gun) is off of Rodimus completely he's sees his opportunity. These musical moments may be absurd beyond any degree of imagination, but they are all-encompassing to those directly involved...and at least in this case, a particuarly excellent distraction. Rodimus turns his attention to the nearby Soundwave who is uncharacteristically distracted himself...if he didn't know any better, Rodimus would say he almost looked positively horrified.

It only takes Rodimus a split second to formulate and put to action a really shitty plan - injured and slower than he'd like to be, Rodimus is amazed Soundwave doesn't see him move, and takes the opportunity to kick the phenomenally distracted Decepticon with as much force as possible in the lower legs. Mr. Miyagi would not be proud. He doesn't stop to make sure Soundwave goes down, but bolts past him, already channelling energy into super-heating the blaster chamber on his arms. He may not be able to fire while handcuffed but there are plenty of other uses for that energy. He's only got a half-formulated, and less-so-by-the-second plan in his head as he makes a move toward Orion and Megatron's hopefully-complete duet.