I agree with diesel vdub and think you need to try the iPhone without the Lifeproof Case and with the headphones directly connected to the headphone jack. When the headphones are removed does the main speaker turn on?
if Yes, without putting the iPhone back in the Lifeproof Case, connect the headphones to the Lifeproof "Adapter" and plug that adapter's pin into the iPhone jack. When you unplug that, does the main speaker turn on?
if Yes then the issue is more with the Lifeproof Case in combination with the adapter. If on the other hand the main speaker doesn't turn on then the adapter pin is somehow causing the iPhone jack to continue to appear in-use.
at this point if powering the iPhone off and back on clears the issue and the main speaker is active then there is something subtle about the adapter pin that is causing the removal to remain undetected to the jack and iPhone software unless powered off and back on.
Think I would invest $7.50 to purchase a second Lifeproof adapter and see if that solves it.