Flat is the opposite of Rough. Rough is related to Texture. Texture is related to Smooth. Smooth is related to Slide. 
Boy is related to Gender. Gender is a synonym of Voice. Voice can be Beautiful. 
Flat <--Antonym--> Rough <--RelatedTo--> Texture <--RelatedTo--> Smooth <--RelatedTo--> Slide 
Boy <--RelatedTo--> Gender <--Synonym--> Voice --HasProperty--> Beautiful 