That old black magic has me in its spell,
That old black magic that you weave so well.
--Harold Arlen and Johnny Mercer, "That Old Black Magic"
Sorcerers are humans who have learned the art of sorcery, by which they learn rituals to project their Will on the Symphony. Because it takes a certain amount of arrogance and selfishness to push your desires on the rest of reality, most sorcerers are in the service of Hell whether they know it or not, and many gained their powers directly from a demon.
Like a Soldier, a sorcerer has at least 6 Forces. Unlike them, sorcerers rarely serve an angel or demon directly, but think of themselves as their own masters. They can hear the Symphony, but rarely do so clearly, often referring to the distorted theme as the "Cacophony" that their own Will brings order to.
Sorcerous rituals often work in a different manner than Songs. Many of them involve either summoning or banishing spirits, often Ethereals from the Marches, but sometimes actual demons. No sorcerer has ever worked out a ritual to summon, banish or command an angel, and many believe it impossible.
A small minority of sorcerers use their powers in the service of an Ethereal, or in partnership with one. An even smaller number work directly for Heaven, the so-called "white sorcerers" that Hell calls a myth.
In a canon In Nomine campaign, sorcery is an almost exclusively human art. The only celestial known to be able to practice it is Hatiphas, the Demon of Sorcery, a high-ranking servant of Kronos. It is thought that Lilith, the only human "Demon Princess" may have some skill at the art as well.