说话轻一点 和 说话大声点, 怎么用英语说?

说法可以是多种多样的...比如:
说话轻一点Lower your voice, 一般不用soft表示声音的大小，它可表示声音的柔和.
说话大声点 Speak louder或者直接说louder!

speak softer
speak louder

speak softly
speak louder

Keep it low!

Please raise your voice!

speak a little softer
speak a little louder

Lower, please!
Louder, please!