Use 'speak up' to ask someone to increase their volume, especially in noisy places.

Volume Control

'Speak up' is not used for whispering or private information; it implies being audible to everyone in the vicinity.

Not for Secrets

'Speak up' often conveys bravery or conviction when sharing opinions, especially if they're unpopular.

Use 'speak up' in situations where there's a risk involved in stating one's opinion.

Use 'speak up for' to show that you're defending someone else, not just yourself.

It implies advocating for causes or principles, not just personal interests.