Singing Crystal Mines
Sorin stands to the east side of the first cavern you come to once you enter the mines.
Some of Sorin's quests are part of the Miner Job Quest Series. He will not offer you any Miner quests until you have completed all previous quests in the series.
Sacred Grove - Chapter One:
Sorin is one of a few npcs that unlocks the Sacred Grove Quest Series - Chapter One .