A missing feature that I really want added to all home audio systems: the ability to listen to multiple audio sources at the same time. I almost always listen to music when playing video games (with the music disabled in-game), listening to podcasts, watching YouTube, and watching sports.
Juggling multiple devices to listen to multiple audio sources at the same time isn’t too much of a pain, but if audio could be easily spliced together on one device that would be awesome (each source would need its own volume level).
Until such a feature is universally available, some makeshift devices like the design above may be worthwhile. This HDMI cable would function like normal for a game console, but can receive a secondary audio source from a smartphone using Bluetooth. Now a gamer could play Rocket League while listening to iTunes on their home theater system at the same time.
There are many possible ways audio splicing could be achieved and applied, but I felt the above design would be the most popular scenario. Again I would much prefer if “audio splicing” became a common feature in all audio electronics, rather than cobbled together in this fashion.
Dave's Geeky Ideas 
The USB is to power the device!
I had the same problem, not for doing it all at once, but I didn’t want to be switching, so I bought a mixer en amazon (Technolink TC-716)
now I have my PC, TV, amazon dot, and a logitech bt receiver all connected there, and my old trusty aiwa NSX-BL24 as an output
also a MI BOX and a bluray, but they go through the tv, so no need to connect them there