The only trick is is that Brennan's very particular about who he sings in front of, so... I'm his mom, for example, I've only heard him sing twice.
14 seconds sound clip from the Step Brothers (2008) movie soundboard.
You can hear this line at 00:11:01 in the Blu-ray version of the movie.
Quote context
[...]
- Robert.
- Robin.
- Actually, Brennan is a really talented person. He's a very gifted singer.
- I've been called the songbird of my generation by people who've heard me. That good.
- That's funny that you say that, because I can sing too. In fact, I'll sing right now.
- If you wanna get down on these hairy balls, why don't you jump right in?
- It's a crotch party right up in here, why don't you lick on this big joint?
- Stop it!
- That's cute. I remember when I had my first beer.
[...]