Depending on the situation and how to addressed the Audience. I would personally use ...
You were singing loudly in the shower.
-Tu estabas cantando en voz alta en la Regadera
friendly approach slang
Tu estabas cantando escandalosamente en la regadera
Here's the part where you used Escandaloso/escandalosamente,Only when you are going to refer to an individual as lazy(I know people in the field that refer to them as loud as a niece way and supposedly professional)
John is very loudly when it comes work
John es bien escandaloso cuando se requiere trabajar
In this above example you wouldn't use High Voice but rather Escandaloso. I grew at a community where people had little education so individuals refer to others as loud when they really meant lazy . I grew at the outskirt of Michoacan Mexico.