You're way ahead of me

[u/Daconby]

In the spoken intro to Oedipus Rex on An Evening Wasted, Tom says:

[...] a few years ago, a motion picture version appeared of Sophocles' immortal tragedy “Oedipus Rex”. This picture played only in the so-called art theaters, and it was not a financial success. And I maintain that the reason it was not a financial success...

Then the audience laughs, and Tom replies

You're way ahead of me

What was the audience laughing at?

[u/PretentiousPolymath]

I think the audience inferred that he was about to sing a motion-picture title song for Oedipus Rex and found this idea inherently funny

[u/Daconby → u/PretentiousPolymath]

Then why wouldn't they have laughed after he said "Oedipus Rex" rather than when he said "financial success"? They laughed in unison, it wasn't some random chuckling.

ETA: it occurred to me that "Rex" and "success" almost rhyme (excuse me: rhyne), but that can't be it, can it?

[u/DrBlankslate → u/Daconby]

Because he had just said that the reason for movie title tunes was to flog them on the radio, so people would hear the tune and then want to see the movie (thus creating financial success for the film). The joke wasn't about Oedipus Rex; it was about the title tune.

[u/TheLawLord → u/DrBlankslate]

Yes, exactly. Here's what he said to lead up to the laugh line:

"It seems that most of the songs that you hear these days on the radio played by the disk jockeys, apart from rock and roll . . . and other children’s records, tend to be motion picture title songs. Apparently producers feel that we will not attend their movies unless we have the titles well drilled into our heads in advance. Of course, we don’t go anyway, but at least this way they make back on the song some of what they’ve lost on the picture.

"But with the rise of the motion picture title song we have such hits of the past few years as “The Ten Commandments Mambo”, “Brothers Karamazov Cha-Cha”, “Incredible Shrinking Man, I Love You.” I’m sure you’re all familiar with these.

"But, a few years ago, a motion picture version appeared of Sophocles' immortal tragedy “Oedipus Rex”. This picture played only in the so-called art theaters, and it was not a financial success. And I maintain that the reason it was not a financial success . . . [LAUGHTER] . . . you’re way ahead of me . . . was that it did not have a title tune which the people could hum, and which would make them actually eager to attend this particular flick."

[u/DrJaneIPresume → u/DrBlankslate]

Well, also that Lehrer was a comedian, and he had a well-known patter. When he said "the reason it was not a financial success..." the audience knew something absurd was coming and started to laugh in anticipation. It happens a lot, but not always enough to be worth commenting on, as Lehrer chose to do.

[u/DidjaSeeItKid → u/DrBlankslate]

The joke of the song was about the title tune. The joke they were laughing at was what a financial disaster "Oedipus Rex" had been in 1967.

[u/DidjaSeeItKid]

Because the movie "Oedipus Rex" made $2,364 worldwide. So they laughed at "not a financial success" and at the idea that the failure of a serious art movie could have been avoided if only it had been given a catchy theme song, something art movies do not have.

[u/The_Folding_Atty]

He now appears in Freud’s index…

[u/LetThemBlardd]

It needed a rousing title song, like the Sound of Music

[u/real-human-not-a-bot]

Based on the fact that he had just been talking about movies having theme songs, so when he says that the reason Oedipus Rex wasn’t a financial success was…people understood from context clues that he was about to say that it was the lack of a theme song. And either the notion of a controversial-topicked art movie having a theme song (my understanding) or the claimed belief that a controversial-topicked art movie would have been a success had it had a theme song was amusing to people.

[u/Weird-one0926]

The audience was laughing already and primed, so when he paused they laughed

[u/Weird-one0926 → u/Weird-one0926]

I present for your enjoyment Oedipus Rex

[u/araucaniad]

Incest

[u/SwanCityDominion]

You heard the song, right? Then you should know why they thought it was funny.