A Songbird Serenade x Sapphire Shores next-gen i got form EppyMinecart
 
Meet Duel Tone!
 
Duel Tone is the second youngest of the Songbird Serenade x Coloratura x Sapphire Shores family, and she specialises in singing in opera/classical music (think back to music from the 1940’s-1960’s, people like Frank Sinatra)  
Her real spelling of her name is “Dual” Tone, but she changed it to Duel, based off a joke her parents used to say about her when she was younger about how competitive she used to be with her other sisters, joking that she was duelling with them over everything.  
Her voice is very calm and soothing, and I imagine her voice sounding like one of those old black female jazz singers.  
She and Sea Song are also volunteers with their mother to help other ponies with singing/vocal lessons.