Sweet Jamz is a catch-all phrase. It describes either a) music that totally kicks ass--whatever that may mean to you (except the Jonas Brothers, which could never be considered sweet jamz), or b), more commonly, any thing, person, place, occurrence, event, situation, eventuality, or outcome that is considered to be positive.
Etymology: probably the '70s. Brought to current vernacular by the residents of Mod 30.
your posse: "yo cindy's throwing a tag-team ladies bikini jello wrestling party this weekend."
you: "sweet jamz dude."
your boss: "we'd like to promote you to executive vice president of corporate development."
you: "sweet jamz yo."
your ma: "hey, wanna hear sleigh bells' 'crown on the ground'?"
you: "hell yeah, ma...SWEET JAMZ!!"