This is a secret Achievement, which is not listed before unlocking and is awarded automatically when The Protagonist sings along with a song on the radio.
Each voice option for The Protagonist can sing two songs, except Male Voice 3 which can sing three.
All songs available appear on 107.77 The Mix FM, and must start at the beginning in order for The Protagonist to sing along and earn the achievement. There are no special requirements and no specific way to trigger it, apart from listening to the correct radio station.
As each of the 6 voices available sing "Take on Me". The easiest way to obtain the achievement is to create a custom playlist that only includes "Take on Me" and drive around. The Protagonist eventually sings along.
In addition to "Take on Me", each voice actor/actress chose a second song which also unlocks the achievement.
Unrelated to the achievement, Pierce sings "So Sick" by Ne-Yo. The Protagonist does not sing this song. The only track in the game files is Pierce.
In several missions with Pierce in a vehicle, The Protagonist comments on his choice of music, and in the Mission Bonding Experience, Pierce changes the radio and sings along to "So Sick" by Ne-Yo. The Black Male Protagonist voice says "Son, you're no Ne-Yo", after Pierce has finished singing. The Hispanic Male Protagonist complains that listening was "worse than getting shot" after Pierce has finished.
- Although this achievement does not appear in Saints Row: The Third, the mission "Steelport Here I Am" features Pierce and The Protagonist singing along to "What I Got" by Sublime.
- In Saints Row IV, Pierce and The Protagonist sing along to "Opposites Attract" by Paula Abdul together as well as "Just A Friend" by Biz Markie, which is interrupted by Zinyak.
- In Saints Row 2: Ultor Exposed, when Tera is called as a homie, she may refer to the Soprano achievement by agreeing to come if The Protagonist promises to not sing "Take On Me".