The concept behind AudioMod is pretty simple. If you're going to add a Mophie-like charging case to your iPhone, you might as well add a headphone jack, too, because you're covering up the Lightning port in the process. In other words, it's kind of a three-for-one accessory (yes, it protects your phone, too) that might have some appeal to those who lament the loss of the headphone jack in recent iterations of the iPhone.
The AudioMod comes in versions for the iPhone 7, 8 and X. All cost $70, which is less than what a Mophie case with an integrated 4,600mAh battery might cost, but more than your typical battery case from a generic brand.
The case seems pretty well-designed and is fairly slim. Encased, the company behind the product, claims the AudioMod is backed by a 5-year warranty. Presuming that's true that's a very good warranty. I only say that because although the case worked fine for the week I used it (it was able to fully charge my iPhone X), I wasn't totally confident it would hold up over many months. I have nothing empirical to back up that assertion, it was just a gut feeling.
The case is equipped with a built-in DSP and DAC chipsets "that serve to amplify and digitize the signal." Encased says, "Jargon aside, this means you get premium audio quality usually reserved for high-end expensive headphones."
I'm not sure exactly what that means, but I hooked up some decent Beyerdynamic headphones and they sounded decent through the headphone jack but not really any better than what you'd get by connecting the Lightning-to-3.5mm dongle to your phone and listening that way. In other words, I didn't feel that I was listening through a premium headphone amp.
It's also worth mentioning that you won't be able to charge your phone wirelessly if you put this case on. That's not the end of the world, but it would be nice if it had that ability.
Overall the AudioMod for iPhone X is an intriguing concept. I'm not sure I'd be willing to pay $70 for it, but it does get you your headphone jack back and has the additional benefit of being a charging case. Alas, like with other Mophie-style cases, it does add some significant bulk to your phone. For a lot of people that extra bulk will only be worth carrying if you really need the extra juice on the go.