3) Yes, "if" is actually "Compare variable" condition.
4) Select "Compare variable" event with mouse and press X, an "Else" condition will be added under it.
5) "Stop" is an action of Audio object. You might want to stop previous sound before playing the next one, to prevent sounds overlapping.
Every sound you play has a tag. If you want, you can assign different tags to each sound. But I guess in your case it's easier to use the same tag.