vocal masturbation 

when you are supposed to be singing a duet, but you instead sing a solo, as coined by Glee

Ken: "I am going to sing my duet with the one person who shares my musical talent and enthusiasm... myself."

Angelina: "But isn't that like vocal masturbation?"

vocal masturbation by music_tenor_skillz March 22, 2011

Vocal Masturbation 

for music production gurus. Harmonizing to your own melody part of a song when in the studio.

After Steve recorded his lead vocal part, vocal masturbation was in order.

Vocal Masturbation by chandlermcmahan February 24, 2010