I don't think they ever watched a wedding video in the office. I remember Jim and Pam watched videos of bands to play at her wedding, but that's about it. I think the post must be wrong...or could it maybe be a deleted scene?

That is probably referring to the scene in "Casino Night" where Pam and Jim are watching the videos of bands who might play at the wedding. There is one of Kevin's band and Jim threatens to tell Kevin he can play, to which Pam grabs his arm to keep him from leaving.

-------------------------------
Jim: [Jim ejects a videotape from the VCR and puts in a new one] Wow. I don't know how you're gonna decide. They are all extremely good.
Pam: I think I should hire them all. Do like Lollapalooza.
Jim: Yes.
Pam: Have three stages, yeah.
Jim: Your mom would love that. She would. Oh, this band is called Scrantonicity.
Pam: Oh.
Jim: Let's take a look. Nice.
Pam: Oh, wait. That's Kevin. On the drums.
Jim: What?
Pam: On the drums! On the drums!
Jim: Oh, my God, that's Kevin! Great song, Kev. Oh, my God, he's the drummer and the singer.


Kevin: We really don't do a lot of weddings. We actually don't play in public very often. We are all really hoping that Pam's wedding works out. This could be a turning point for the band.


Jim: Wow. Oh!
Pam: Oh, my...
Jim: Yeah, you haven't seen that since 1983. That is amazing. Okay, we have to sign him. I'm gonna call the label, we're gonna...
Pam: No! No! [Pam grabs his arm and pulls him back]
Jim: No, Pam, you're gonna loose him to another wedding.
Pam: No, come back! No, no, no!
Jim: Kev!