Is there a spell that makes things get louder?

Sonorus is the spell. Quietus is the counter-spell. Otherwise you may invent one (you may imagine things in fanfic !). For your future reference, you may check the HP Lexicon, very useful too (I use it as well, as a fanfic writer myself). Leave a link to your story once it's completed and posted.

Sonorus makes JUST your voice louder and Quietus reverses the spell.

I haven't found one for amplifying things in general. You could always use the extendable ears that Fred and George sell in their shop.

Yup, it's Sonorus. I wonder though, is that what Voldemort uses in DH? :)) Seems kinda lame of him not to use a different DARKER spell, if you know what I mean.