[Beethoven's Symphony No. 9, Fourth Movement - An Die Freud][1].<br>
^^ Full lyrics and English translation are on the Wikipedia page linked above ^^

And here's a video with lyrics and English translation following the symphonic version:


This track and much of the synthesizer music on that soundtrack was arranged and performed by [Wendy Carlos][4], who is probably best known for "[Switched-On Bach][5]".  She went on to work with Kubrick again, composing much of the soundtrack for "The Shining".

  [1]: https://en.wikipedia.org/wiki/Symphony_No._9_%28Beethoven%29#Fourth_movement
  [2]: https://www.youtube.com/watch?v=4pbMUEHvoAo
  [4]: https://en.wikipedia.org/wiki/Wendy_Carlos
  [5]: https://en.wikipedia.org/wiki/Switched-On_Bach#Influence