A few weeks ago I switched from Lollypop to cmus and tried to get my media keys to call cmus-remote to control it. After I couldn't get it to work I sort of gave up on it. Yesterday I pressed one of the keys and they suddenly worked. Turns out the latest cmus updated added MPRIS support and now they work out of the box. I love it when problems are fixed by just being a little patient.
Also big thanks to anyone helping to get these updates to Solus.