I’m still not sure.
I kept asking my lovely AirBnB host to pronounce it over and over. It sounded so easy coming from a native Norwegian but I couldn’t quite grasp it.
Some people just say Tromso but the letter ø in Norwegian makes things sound different. I know it is a mixture between the letters A and O, pronouncing it like “oh” but with an A in front of it.
Does that make sense? Because it still doesn’t to me.
Give these version’s a listen: http://www.forvo.com/word/troms%C3%B8/