Hello,
I currently use the NGP. The case is excellent, slim yet it does protect drops(my phone falled from 3m and nothing!) and scratches. As for your main question, yes the audio cut out is quite small. My bose Mie2i and shure 215 fit but not so perfectly like the apple stock/in ear IEM's. As for my shure, the stock cable that came with the IEM's DOESN'T fit, but since i also bought an aditional cable with mic, that DOES fit because the jack head diameter is smaller than the non-mic cable. Shure 215 have detachable cables for me to change them.
The design itself is not flashy or "sexy" in fact im already bored of it even though is a "perfect" case for me in terms of protection and profile(slim). If you're set on the NGP, I think a translucent blue would really good

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As for a solution, I strongly advice you to check the Belkin 050 and SGP Modello series. Both cases are made of TPU like the NGP. As i've seen in some pictures, the Modello audio cut out seems to be quite bigger than the NGP but still i think it can limitate some aftermarket headphones/IEMS jack head.
As for the Belkin 050, I haven't seen its cutout therefore I can't give you a precise answer.
All in all, I think i will buy the Modello aswell. You should check it!
PS. tell me what aftermarket headphone/IEM you have/want and ill check if it's jack head is compatible with the NGP audio cut out.
Hope I helped! and sorry for any mistake while writing
