Thanks mate that helped heaps and I was able to delete the sound. It didn't however help me find the sound originally. So if you forget what layer the sound is attached to, I guess you just have to go through each item.
For future reference, if you click the eye icon at the top-right of the Timeline it opens a filter window that lets you choose what type of tracks should be displayed. Just turn off everything but sound and that should lead you to the culprit.