This is not quite what you want but might be simpler to implement. Instead of choosing the file when the SMS arrives, choose it for the next SMS that will arrive. A casual user won't notice any difference.

Now how do you do that? You can employ some dbus magic but the simplest approach is, run a cron job that will periodically link the SMS notification file to the next file in your folder. If you receive SMS randomly (as most people do), the effect will be a random file played on the next SMS.