Hecate, the ruler of the three witches in Shakespeare's The Tragedy of Macbeth, is named for the Greek goddess of witchcraft. In Greek, Hecate is spelled with a k—as in Hekate—and the name is pronounced [he-KAH-tay] or [he-KAH-tee], with the stress on the middle syllable. In Shakespeare's English, it is often pronounced [HEK-ut] or [HEK-ah-tee], with the stress on the first syllable.

Siward, the English general and Earl of Northumberland who leads the English forces in the battle against Macbeth, is pronounced [SEE-werd] and sometimes even [SEE-erd]. Often, the sound of the r is dropped completely, as in [SYOO-uhd]. In all variations of the name's pronunciation, the emphasis is placed on the first syllable. Young Siward, who is killed in battle, is Siward's son, so his name is pronounced the same as his father's.

Good ole' Webster provides another possible pronounciation for Hecate: heh-kah-tee and heh-ket

How's that for multiple directions? - just about anything you say is correct!

I have also heard the name Hecate pronounced various ways--in Macbeth it fits the iambic rhythm with the bisyllabic HECK-et; however, according to Enclylopedia Mythica found at the link below, the correct pronunciation is Hecate {hek'-a-tee} Greek, which places the accent on the first syllable.

As for Siward, the only pronunciation guides I could find all agree with the SEE-ward pronunciation.

Siward is pronounced SEE-ward
I've heard Hecate pronounced two ways: Heck-ate and Heah-KAH-tae
My wiccan friends pronounce it Heah-KAH-tae, as do I. This pronunciation is closest to the Greek, but I've also heard it said that her name is disyllabic in the play, and I have frequently heard the first version. To complicate matters further, Shakespeare often left out parts of names to make them fit or rhyme, so this is definitely not a definitive answer.