(Emoji art supplied by Emoji One.)
Creating the community wiki as suggested:
1. Sleeping + Cash = Epica
2. Exclamation mark + Grandfather + Theater = Amaranthe
3. Search + Montenegro + Myanmar = Arch Enemy
4. Kazakhstan + Camel + Hot = Kamelot
5. Leaf + Vessel + Eyes = Leaves' Eyes
6. Delicious + Rain = Delain
7. Switzerland + Thinking + Temple + Station = Within Temptation
8. Poison + Qatar + Archery + Vertical = Sonata Arctica
9. Night + Switzerland + Ash = Nightwish
A. Wheel + United States + Water + Spade = White Waters
B. Satellite + Cricket + Memo + Kenya = Sacrimony
C. House + Noon + Astonished + Chilli = House on a Hill
D. Wedge + Coffin + Fist + Wheel = Edge of Steel
E. Theater + Train + Gem + Ladybug + Coffin + Theater + Comet + Lemon + Spade = The Tragedy of the Commons
F. Astonished + Day + Fortune + Ghost + Spade = A Day for Ghos(t)s (I'm thinking Sleafar miscounted this one)
G. Thunder + Grin + Key + Skier + Spade = Under Grey Skies
H. No + Computer + Lips + Candy + Ice cream = No Compliance
I. Spark + Sad + Sunrise = Paradise
J. Castle + Spiral + Romania + Ice cream = Astral Romance
Solid green line: connects a band to its song.
Dashed blue line: connects a band to a song where one of its members provided guest vocals.
Dotted purple line: connects a band to a song where one of its members provided guest instrumentals.
Example: Under Grey Skies (G) is by Kamelot (4) with some vocals from Charlotte Wessels of Delain (6) and tin whistle played by Troy Donockley of Nightwish (9).
Ok, pretty sure I've cracked the code. Mostly.
The green numbered items are bands, and the blue lettered items are songs. The solid green lines connect bands to their songs. Not sure what the other two types of lines represent. Musicians from other bands guesting on a song? A producer connection? I'm not sure because I haven't solved all of the clues yet, and Metal is not a genre of music that I'm super familiar with. (This is a headbanging puzzle, remember.)
represent skip and take. So, dots on the left side of an emoji mean skip that many letters, and the dots on the right side mean take the next x number of letters. If there are no dots on the left, you just take from the beginning.
So, for example, starting with number 1 you have:
which gives you EPICA,
which is connected to letter A:
Epica's song White Waters.
HOT (tea? coffee?)
Letter B (this is the first one I got, actually):
Under Grey Skies
there are no solid green lines from Arch Enemy, so I am assuming there are no songs from them, and I don't know what the relationship is between Arch Enemy and the song Sacrimony.
So that's all I've got right now, but I think it all pretty much fits. I will try to solve some more tomorrow. Maybe someone wants to create a community wiki answer where we can all contribute answers for the rest of the letters and numbers?