“You sound horrible!”
You're talking to a friend on the telephone. Her voice sounds deep and scratchy. You say this because it sounds like she's sick.
You sound horrible!
There are two meanings of "sound (adjective)". One is "Your voice sounds (adjective)." The example above has this meaning. Another example is if you sing a song, and afterward someone says:
You sounded great!
The other meaning of "You sound ___" is "You sound like you are (adjective)":
You sound tired.
You sound excited.
This is what people say to you when you're sick and your voice changes.
The word "horrible" means "really bad". So telling someone that they sound "horrible" can be taken as an insult. If you say that your friend sounds horrible after she sings karaoke, it's really mean. But when someone is sick, telling them "You sound horrible!" expresses your sympathy for them.