It's actually pronounced like it's spelt, Nih-sah, with a short 'i' sound. Hopefully I would know seeing at it's mine, unless I've been mispronouncing it all these years. XD
I'd say Miss Claire, who is a Hebrew speaker, would list the correct pronunciation. I'd go with people who have credentials. From what I've gathered, "i" makes an "ee" sound in Hebrew. The way you say Nissa is probably an anglicised pronunciation.