Xiaomi eliminated the conventional earpiece hole because it wanted the screen to utilize the largest amount of space at the front. Adding a hole at the top will require a thicker bezel and putting a hole at the top of the display is obviously out of the question.
Xiaomi Mi Mix utilizes a process that transmits an electric signal from the Digital to Analog Converter to a piezoelectric ceramic which turns the electric signal to a mechanical energy which reaches the phone's frame, vibrates the metal frame and produces a sound that reaches your ear. This is innovation at its finest and Xiaomi had to solve many issues due to the unique phone design because it couldn't use current technologies, that just wouldn't work with their design principles.
According to Xiaomi, people can hear other people better in very noisy environment. I am eager to try it myself and see how the call quality really is - I'm pretty sure it won't disappoint.
Here's an image that shows how the convertion process works:
This Cantilever piezoelectric ceramic acoustic technology was what caused Xiaomo to use the ceramic body, although it is very expensive to produce (~RMB 1000). It's the first phone that utilizes such advanced acoustic technology and I'm sure that this technology will be used in the future because it helps hide some parts and help create phones with larger displays.
This is exactly one of the things that makes me admire Xiaomi, because it pushes technology innovation forward. We haven't seen any innovation in the mobile phone market in recent years, and that's why I am so excited to finally see a company who put a goal to innovate and move the mobile phone industry forward and create technology innovations that can help create better smartphones.
Image credit: Xiaomi