when I was a kid, I always remembered the difference by thinking of the line as a pipe that lets electricity flow, and the circle as something blocking that pipe (and therefore blocking the flow)
Sadly, the symbols mentioned in the article aren't always used the way they should. For instance, earlier today there was a link on HN about an HTML5 audio player, Speakker [1], which uses the standby symbol as a play button. Confuses the hell out of me.
Either way I wanted to thank you for leading me to some very awesome music which is available under Creative Commons!