This name is a favorite of the Dutch royal family and has also been born by many a Dutch noble. Personally, I like the name, especially the way it ends in -ix, but I can never get past the snooty, pink-polo-wearing, upper-middle-class vibe I get from it, so I would never use it.
Derived from Philips of Marnix, Lord of Saint-Aldegonde (1540 – 1598), commonly known as "Marnix van Sint Aldegonde", so his surname was perceived as a first name. Marnix itself is a town or region of unknown meaning and location.
The IPA-pronunciation of Marnix should be: /'mɑr.nɪks/ [noted -ed]
If need be, you can also listen to the audio clips featured on the websites below:
• https://nl.wiktionary.org/wiki/Marnix (in Dutch)
• https://nl.forvo.com/search/Marnix/nl/ (in Dutch)
• https://youtu.be/UXWBAHLpTvk (in Dutch; the name is said at 0:01)
• https://youtu.be/wOuMddffymM (in Dutch; the name is said at 0:02)
• https://youtu.be/Pj6sGUxY3nY (in Dutch; the name is said at 0:02 and 0:15)