roslynmuse and Il Grand Inquistor are very close, would just like a list of the four pieces in question (and, where a piece consists of a cycle, the specific chapter(s)), and the reference in question.
Ok, Ian, this is a tough one. Here are my answers as fully as I can find them (having little or no German made researching the Schnebel tricky)
Ives –
Piano Sonata No.2, Concord (each movement quotes the opening bars of Beethoven’s 5th Symphony)
Walter Murphy -
"A Fifth of Beethoven" - disco version of Beethoven's Fifth Symphony
Finnissy -
History of Photography in Sound – Concert 2:
Capitalist realism (with Sicilian male nudes and Bachian paraphrases) quotes Beethoven’s 5th Symphony as well as String Quartet Op.18 No.5 and Piano Sonata Op.10 No.1
Dieter Schnebel - Beethoven Symphony (1985) (Re-Visionen I, no. 2) for percussion and chamber ensemble...no idea which piece of Beethoven it quotes, but as the others quote the 5th, I’ll go with that one as the link.
Fingers crossed!!