I first encountered her in slightly different form as Semirama, a character in Roger Zelazny's The Changing Land (the second of the two books about his character Dilvish the Damned, which is itself a sort of epilogue to William Hope Hodgson's The House on the Borderland). I can't lay hands on my copy at the moment but she was rather memorably brought to life the illustrator; in the book, she is an ancient queen, resurrected centuries after her death to help contain the mad deity at the centre of the story.
I have no idea how Zelazny intended her name to be pronunced, but I automatically read it as along the same lines as semiCOLon or semiFINal, thus "SemiRAMa" or in the traditional version "SemiRAMis". (I never really thought about why she would be half of a Rama or Ramis, though.)
However, it's absolutely clear that Shakespeare has a different pronuniciation in mind:
To wait, said I? To wanton with this queen,
This goddess, this Semiramis, this nymph,
This siren, that will charm Rome's Saturnine
Ay, come, Semiramis - nay, barbarous Tamora,
For no name fits thy nature but thy own!
Or wilt thou sleep? We'll have thee to a couch
Softer and sweeter than the lustful bed
On purpose trimm'd up for Semiramis.
I guess it wouldn't have occurred to me so quickly if I was just reading the plays rather than listening to them as well, but it's obvious that Shakespeare is stressing the antepenultimate (or, as we say in English, third last) syllable: "SeMIRamis".
Wikipedia, as so often, has much interesting information about Semiramis, including that her original name may have been the Babylonian "Shammur-amat". Ancient Babylonian is not one of my languages, so I don't know where the stress would be in "Shammur".
Then again, when you consider how different the modern pronunciation of "Julius Cæsar" is from the way he probably said it ("Yoolius Kaiser"), Semiramis would probably be glad to know that she is remembered at all this long after her death, and not too worried about the pronunciation of her name by people in countries she did not know existed.