You don't even need a script for that!
This can be done using Plus!' Event Notifications.
- Open up your contact list
- Click the Plus! button and select Preferences & Options from the menu
- Go to the tab Customize (above) and then choose Event Notifications (left)
- Click Add to make a new event notifications
- Check the contacts you want the notification for and click Next
- Choose "User has signed in" from the list and click Next
- Check "Play a sound when this event occurs", browse for the sound file, set the maximum play duration and click Next
- Choose "This event will never expire" and click Done.
If you ever want to change or delete this event again, go back to that Preferences page, select the event and hit Edit/Delete.
I don't know whether this sound will either override or blend with the default sign-in sound from Messenger. However I think that whenever someone you selected signs in, you'll certainly here the difference.
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