Why is the voice compared to a snake?
Rashi citing 32 Midos of R. Eliezer Bno Shel R. Yosi ha'Gelili, Midah 12: The snake comes to teach about Egypt, and it is learned from it! We learn from here that Hashem told the snake "Al Gechoncha Telech" (Bereishis 3;14), He cut off its legs, and its voice (of pain) went to the end of the world.
Radak: The voice of Egypt's screaming will go far, like a snake's voice.
Radak citing Targum Yonasan: The sound of banging their weapons is like a creeping snake.
Malbim: Just like the snake had legs and they were cut off due to its sin, so Egypt's legs were cut off; they cannot go outside their land.
Why does it say "v'Chayil Yelechu"?
Rashi: They will go to [Egypt]. Malbim - they go with axes, to cut off its feet.
