I just recently got a pair of Astro A40's + Mixamp Pro along with the Official Headset Adaptor and set it all up by following some YouTube guides but the microphone just didn't work at all so I tried for 2 hours fucking about with the Xbox one's settings, Astro A40’s Mixamp Pro and Official Headset Adapter and I still couldn't get it work, It was driving me absolutely fucking insane.
I pretty much gave up and started putting everything back in its box so that I could take it back to the store GAME UK that I got it from so that I could get a refund for it and pick up a different headset instead such as the Turtle Beach Stealth 500x but I decided to Google the issue and I quickly saw that a lot of people on Astro Gaming Forums have been having very similar issues and quite a few people were recommending this 1.2m 2.5mm - 3.5mm chat cable and a few people said that this chat cable completely solved there issues.
I decided to take the risk and buy the Chat Cable considering I already spent £200 of the Astro A40's which at that time didn't work with the Xbox one and the Official Headset Adapter so another £10 wasn't a big deal and amazingly the chat cable turned up the next day.
I plugged the 2.5mm end into the Astro Mixamp Pro’s chat port and the 3.5mm end into the Official Headset Adapter and I've got to tell you that it fucking work’s and I am now absolutely fucking buzzing that my pair of Astro A40's now work perfectly fine with my Xbox One thanks to that chat cable but just remember to have the Official Headset Adapter set up so that the top right button ( + ) is set to max so that it beeps and also have the bottom left button ( - ) set to max so that beeps too and everything will work nicely.
Hopefully this information helps.