After leaving Yidish-vokh I braved morning rush-hour to drive into Washington DC. It was a rocky start to the day, but the very helpful librarian in the Hebraic Section sat me down with a book called Yiddish American popular songs, 1895 to 1950 - it's a finding aid for the Irene Heskes collection which is housed there in the Performing Arts Reading Room.
I learned that, despite the impression one might get from reading the introduction, not even 2/3 of the songs catalogued in the book are present at the Library of Congress. Irene's collection was about (let's see if I remember) 500+ pieces; somewhat more than 1000 pieces were sent down from the copyright office (when composers wanted to copyright their songs, they sent them in, often on lousy little pieces of notebook paper and half-sheets of music paper); the other 500+ were spotted by Irene somewhere, at some time, but she has not left any clues to where those places were!
I also learned that there were often several songs with the same title. I had some titles I was searching for like "Di Velt Hot Zikh Ibergekert," "Zi Hot Es," and "Nerven, Nerven" and there are songs with those names in the LOC collection - but they're not the right songs.
You have to sit in the African and Middle Eastern Reading Room and take down the Heskes numbers. Then you go into the basement and walk through tunnels for about 20 minutes until you get to the place where you answer a lot of questions in order to get a reader's ID card, and then you go and fill out a separate call slip for each piece of music you want. Then they go away and a while later come back with some of what you want and lots of call slips marked "we don't have it."
You can save a little fruitless anticipation in the reading room if you ask for the annotated copy of the Heskes book - an intern has marked every single entry for pieces of music that they have - if there is no mark, they don't have it, so don't bother asking for it.
You can take pictures. They're friendly. And that's my report.