Something euphonious sounds beautiful and pleasant. "You have a euphonious voice!" is a great compliment for a singer.
This word sounds pretty when you say it, so it makes sense that it describes something pleasing to the ear. Most often, this word applies to music (probably not a heavy metal band, though). A great public speaker might have a euphonious voice. A well-written poem sounds euphonious. Many people find the sounds of nature euphonious. But the sound of barking, howling dogs? Not euphonious.