If someone is whispering, but you can’t hear them, how do you fix that problem? question options: tell them to decrease the wavelength of their sound waves. tell them to decrease the amplitude of their sound waves. tell them to increase the amplitude of their sound waves. tell them to increase the wavelength of their sound waves.

Technically, what you want is for them to increase the amplitude
of their sound waves. 

But chances are, if you SAY it to them in that way, they will have
no idea what you're talking about, they will think you're pretty weird,
and they'll continue to mutter and mumble. 

So, although it's not technically elegant, you're better off just saying
"Hey ! Talk louder willya !"