Without testing, my first thought is: don't run this in Parallel. Parallel events must be used with caution, even outside scripts. In a Parallel event, it's re-running the two commands back-to-back constantly.
Furthermore, the load command takes a couple of seconds, so running Play immediately after it (even if not in Parallel) will likely cause desynching weirdness in variable mix play. You should generally load the music before you need it, by at least a few seconds. In another event, if need be.
"Auto" events block the normal game flow from running, so you need to turn off whatever switch activates it at the end of the event. However, I don't think either one is what you're looking for here. It sounds like what you really want to do is just have a "None" trigger common event, and call it with the "Common Event..." command.
If you're still having problems after all of this, let me know.