If it's intermittant, it could be the connections or the battery is weak. It's easy to clean & check the connections. Especially to be sure the ground wire is still connected....that could make it weak.
What year is the PT? The beep on locking is just a short peep, compared to a regular blast when the engine is running; and could be because the horn itself is older.....not brand new.
(just some thoughts....having installed the second "high tone" horn myself.