Strange Problem: Only Rings for Texts When Headphone Jack is Occupied

[ftloosenfanzfree]

A while after getting my phone, it randomly stopped ringing when I got text messages.  It would still ring when I get a phone call and all other sounds worked fine.

I had to reset my phone at one point because my father set it up for me under a different email and I wanted my phone to be associated with my main email.  After resetting, the text message ringtone worked again.  A little while later, it again stopped for no reason.  Everyone just kept telling me to check the text ringtone setting, even though it was definitely set properly.

In December, I was driving to visit family and had my phone plugged into the car's AUX jack so that GPS directions would come in through the car speakers.  I was so startled when I heard my text message ringtone for the first time in so long!  I thought it had randomly fixed itself and was so excited.  But then I stopped hearing it.  That has happened a few times since then.

Last week, I had my headphones plugged into my phone because I had been listening to an audio book and I forgot to take them out when I was done.  The volume was loud enough that when I received a text message, even though the headphones were not in my ears I could still hear the text ringtone.

So now I realize that for some reason when there is something plugged into the headphone jack, the text ringtone works properly, but otherwise I only know I have texts from the visual notifications.  Again, all other ringtones and sound notifications work.

Does anyone know what could be causing this?  I quite like my text message ringtone (the first few notes of the Bad Horse Reprise from Dr. Horrible's Sing-Along Blog!), and I often don't notice I have texts for hours.