How do I lower the hardware volume? (volume too high)

I doubt, there is anything you can do in software. If other earphones have usable volume levels, the output circuit is likely just not designed for low impedance headphones. As a workaround you could use an extension cable with a integrated volume control so you can use more appropriate levels in your digital mixer.

Another (obvious) workaround would be to use another audio interface connected via USB. Also apart from the high volume this could be interesting for your project, since integrated audio outputs tend to have rather poor signal quality anyway. I however cannot recommend any specific brand oder model.