How can I convince my students to speak louder? Freshers sometimes speak almost inaudibly, even after asking them to speak loudly. Now, with facemasks, it's even worse, I am not allowed to approach them too much, and it demands a lot of energy to be able to hear them sometimes. What strategies do you suggest?
If students do not speak loudly when directly asked to speak loudly, it probably means they do not know how to speak loudly.  This is not intuitive because most people think everyone knows how to speak loudly naturally, but I suspect many people misunderstand the mechanics of speaking loudly.  In particular, people think they speak loudly by moving their ribs, when actually the need to move their abdominal muscles.  Show your students where those muscles are and have them practice moving them to make a loud sound.  This will help students control their volume.
You might also ask the music or theater department for advice.