Why is this quote metonymy? "Her voice is full of money."?
The "original" word describing her voice (ex. "rich") has been replaced with an expression to indicate rich - in this case "money".
Metonymy is a figure of speech that replaces the word/phrase being indicated with a word/phrase with which it is closely associated. For instance:
"My love lies sleeping" - "love" refers to a loved one, spouse, something of that sort.
"Let's keep the lid on this" - "lid" refers to keeping something secret or private.
Moving to the question, we have:
"Her voice is full of money".
The question has replaced the word to describe her voice with the word "money". What could it mean? I have to admit that this is an expression that I'm not familiar with, but I can take a guess or two:
Her voice could be quite rich and full.
The way she speaks may indicate she is wealthy.
If I had to choose, it'd the be the first one.