How should I hang speakers with a keyhole mounting slot?
However you like, as long as there's a suitably-sized screw head present. 

That slot you see is designed to accept a screw with a head (or similar stud) that fits the wide part and a shaft that fits the small part. As long as you meet those requirements you're good. 

Whether you facilitate wire hiding is up to you. Low-voltage boxes and plates can work well, or simply a round grommet set into the wall. Be sure there's not a stud where you cut the hole.

I'd consider putting some felt or silicone pads on the back of the speaker to reduce vibration noise and finish damage.