Try turning off the bluetooth on your phone to begin with, then turn off the headphones, and finally disable the bluetooth on your computer. Once everything is turned off, turn the computer's BT back on and wait for it to re-initialize. Turning the computer's BT off and back on seems (for me anyway) to reset all the audio hooks. Go in to the computer's BT settings and delete the M2 device so it can be found again. Once deleted, turn your headphones back on and force it to search and re-pair with the computer. It seems to take forever for them to pair but wait it out until the computer sees them again and discovers them by their correct name. On my Win7 Pro boxes, the headphones will be found and then you have to go through the excruciating ordeal of windoze screw-up, er, Update, find all the drivers. I believe it found 6 different drivers to install and failed on the last two. Those failures are functionality that apparently only phones have. Anyway it
should successfully complete installation of everything else. When I go into to the bluetooth devices and double-click on my headphones, I have two "Supported Audio Services" - "hands-free headset with microphone" and "stereo headphones". That seems to be good enough to get me going. I get the basic functionality of pause/resume and next/previous song (I think - the last two seem to be hit or miss).
The keys for me to get both these and my PXC 550's working on my computer are to first disable the BT on my phone and to disable/re-enable the BT on the computer. While the headphones will work with BT enabled on the phone and computer, I've found that if the phone gets an email, I hear that, but then the music that was coming from the computer has stopped because the connection was broken. I then have to disable BT on both systems, turn on the computer's BT ONLY, and then manually re-pair on the computer side. If I haven't used the phones on the computer in a couple days (I shut my home system down when I'm at work), I find that they won't pair and trying to get them to connect manually doesn't work either. I found I have to disable and re-enable the computer's BT and then they connect up - sometimes I have to do it manually the first time, but turning the headphones off and on afterwards will see them pair automatically.
I've searched for APTX drivers for the computer (to get rid of the annoying driver errors shown in the Device Mangler) and have found some, but they wouldn't install on my two Dell systems, saying newer drivers are already installed. Even searching Dell for new drivers came up empty. And for what it's worth, I bought a cheapo USB BT dongle off of Amazon for my work computer and my home system has BT built-in. Sennheiser has a dongle on Amazon for something like $50 but I don't think it will get you any extra functionality, so I wouldn't waste your money.
Good luck! I don't understand why all this USB/BT connectivity has to be such a pain in the arse. It's touted as being so easy to use. Yeah, if you're lucky on the first shot, great! But if it breaks, you're screwed!