Consistent failure of THAT 1646 line driver IC. What could be the cause?
I'm not sure this question is well suited to the SE format, but anyway here is one way that the chip could fail due to output abuse. 

If the chip (with the diodes) is plugged into a 48V microphone input that has relatively large capacitors it can instantly drive the power supply beyond abs max limits. 

Usually regulators cannot sink current, they can only source it, so connecting a much larger capacitor to the output will cause the voltage to divide depending on the capacitor ratio. After that it would be limited by the phantom power current.  

This could probably be solved by putting a TVS across the supply, rated to take the highest plausible 48V current. That might require a large package. 

There are other possibilities involving putting something in series with the output in addition to a TVS.