This was discussed somewhat off-topic in another thread, as well. Someone asked how many times the voice was heard. For the third season, the voice behind the curtain is used five times:
Episode 301
Kermit: Is he ready back there?
Voice: Uh, no, no.
Gonzo: I'm ready!
Voice: Oh, yeah, yeah, yeah.
Episode 310
Kermit: Is the set changed yet?
Voice: Yeah, it's okay, Frog.
Episode 311
Kermit: Is she- Is she ready back there?
Voice: Who?
Kermit: Raquel!
Voice: Oh, yeah, yeah.
Episode 313
Kermit: Uh, are you ready back there?
Voice: No, we're not ready.
Kermit: Oh, well, I'll just tell you something about Helen Reddy-
Voice: I said we weren't ready.
Kermit: I wasn't talking to you!
Voice: ... Okay, we're ready.
Episode 324
Kermit: Are we ready back there?
Voice: Who wants to know?
Kermit: I do!
Voice: Oh, oh, oh. Yeah. Sorry, boss.