bk, relative to Sondheim + Tick Tick Boom, I found the following online:
NEW YORK Opening-nighters at the musical "Tick, Tick ... Boom!" were astounded to hear Stephen Sondheim's voice blare out on the answering machine when the Jonathan Larson character (Raul Esparza), a down-and-out composer-lyricist, receives an unexpected call from his idol. At the post-perf party June 13 at Chinghalle, "Tick" producer Robyn Goodman said getting the notoriously private Sondheim to lend his vocals to the show was no problem.
"I faxed him a request and he said yes. He remembered going to one of Jonathan's workshops."
That leads me to suspect that the moment is captured either on the cast album of Tick Tick Boom, which I have, or the Jonathan Larson demo disc, which I also have. But I can't confirm for you at this moment if that is the case for sure.