HT is American country music’s early Honky Tonk

ATS is American Tribal Style

As for MTM, I don’t know. I know there is one. MTM audio track format.

To add, have you misunderstood these three words? There is no MTM in the music style. These three albums are linkpark's three albums

"Hybrid Theory", "Minutes to Midnight", and "A Thousand Suns"