Looking at the other answers, I would like to intervene. Whatever the correct pronunciation in Arabic is, we are talking about English here. Merriam-Webster lists quite a few variations:
\is-ˈläm, iz-, -ˈlam, ˈis-ˌ, ˈiz-ˌ\
It also provides two audio recordings, one for /ɪsˈlɑːm/ and one for /ɪzˈlæm/.
The Wiktionary says:
Pronunciation
- enPR: ĭs-läm', IPA: /ɪsˈlɑːm/, SAMPA: /Is"lA:m/
- enPR: ĭz-läm', IPA: /ɪzˈlɑːm/, SAMPA: /Iz"lA:m/
- enPR: ĭz'lăm, IPA: /ˈɪzlæm/, SAMPA: /"Izl{m/
Again, with all due respect to other languages, we just don't pronounce matador the way it is pronounced in Spanish, sputnik the way it is pronounced in Russian, or kindergarten the way it is pronounced in German.
