When "would" means future in the past or a formal request for something

[Mordant]

"Listen up" and "speak up" are more emphatic commands than "listen" and "speak." All are grammatical.

[Waiyin Cheng]

listen up : pay attention to listen what I say. For example, a lot of students in a class keep talking, do not pay attention what the teacher says. The teacher can say "listen up". It will draw their attention to what the teacher says.

Speak up = speak more loudly.

They are grammatical, using common in speaking rather than writing