This is how you pronounce Phnom Penh, the capital city of Cambodia. But first, it’s important to note the word construction and history. Phnom means big hill and is used throughout Cambodia. Penh refers to Daun Penh, or Lady Penh. Legend has it that Daun Penh found five Buddha statues inside a floating tree along the Mekong River. She took this as inspiration to build a home for the statues in what is now known as Wat Phnom (Temple on the Hill). As the Temple became well known, so did the area now known as Phnom Penh grow in prominence. In 1430, after a Siamese incursion, King Ponhea Yeat ordered that the capital be moved from Angkor Thom to near Wat Phnom.
Daun Penh today refers to the district of Phnom Penh where the story originates and you can go visit the statue of Daun Penh at Wat Phnom.
Now that you know that Phnom Penh refers to the hill that Lady Penh claimed for Wat Phnom, and that Phnom is used throughout Cambodia to designate a hilly locale, this is how you want to say Phnom Penh:
Phnom is pronounced as “Pa-num” and Penh is pronounced not like “pen” as spelled but harder as in “Pa-num pbin”. You would certainly be fine pronouncing Phnom Penh as “Pa-nom Penn” as is most often taught to foreigners but it’s more savvy to say it like the locals, “Pa-num Pbin”, and for extra points, say it fast, not drawn out — like you say it all the time.
The following Khmer video is captioned for English and tells the story of Daun Penh – watch for his pronunciation of Phnom Penh.