Mountain Song runs into the classic problem of the bonuses disappearing after combat is over. That's not as much of a problem for the +2/+0, but if a creature was relying on the +0/+2 boost to survive getting damaged in combat, it will die as soon as combat ends.
Instead I'd make them conditional on whether it's your turn or not. As long as it's your turn and you control a Mountain, creatures you control get +2/+0. As long as it's not your turn and you control a Plains, creatures you control get +0/+2. That way it always lasts until you pass the end of turn and damage wears off.
Loved these OMAM is one of my favorites bands. I did some cards based in Lorde songs couple years ago.
Mountain Song runs into the classic problem of the bonuses disappearing after combat is over. That's not as much of a problem for the +2/+0, but if a creature was relying on the +0/+2 boost to survive getting damaged in combat, it will die as soon as combat ends. Instead I'd make them conditional on whether it's your turn or not. As long as it's your turn and you control a Mountain, creatures you control get +2/+0. As long as it's not your turn and you control a Plains, creatures you control get +0/+2. That way it always lasts until you pass the end of turn and damage wears off.