In reply to Bob Kemp:
You have inspired me to browse a little bit further on avantgarde music. Mainly to support my memory.
In 1987 the Danish National Academy of Music in Odense held what they called a Stockhausen week with the composer himself and his orchestra. It was one of these concerts that I attended back then. I recall a small incident during the performance of a piece named Oberlippentanz for solo piccolo trumpet:
Imagine the whole concert hall listening in concentration and absolute silence when a traditional wind-up alarm clock went off really loud. At first I thought it was part of the composition, Stockhausen being Stockhausen, but it was one of the students among the audience who had it in her bag. She couldn't stop it sounding and had to run out into the foyer not to disturb any further.
Reading about Stockhausen lead me to one of his contemporaries namely György Ligeti. At first I couldn't quite remember where I had heard his name before, but then it struck me: Stanley Kubrick used his music in several of his movies. Most famously, maybe, in the Star Gate sequence of his 2001: A Space Odyssey which is accompanied by Atmosphères: