Of Monsters and Men - Mountain Sound Lyrics Meaning

"I heard them calling in the distance" I also think this has something to do with a wild call of the mountain, something they feel more than hear. Maybe "they" is referred to "people like him", people who belong in the mountain, the outcasts, the ones that always feel like outsiders.
"so I packed my things and ran
far away from all the trouble
I had caused with my two hands" this may have somethinh to do with anxiety or some other sort of mental disorder, they created all their problems, but they were not less valid. This fits with him maybe feeling like an outcast.

"Hold your horses now
we sleep until the sun goes down" this may refer to the fact that now that they ran away (metaphorically or not) they can relax, they no longer have the pressure of being someone they were not and feeling as strangers around others. And now they can basically run deep into the woods and the mountain sound (their scape place)

"Some had scars and some hid scratches, it made me wonder about their past..pasthere I feel like they found the other "outsiders" and they have different stories and their own scars, but they all had one thing in common
"I began to notice that we were nothing like the rest" (they're all different between them) or (they and all the other "weirdos" they found are nothing like the 'ordinary' people...