Most words ending in -um are words derived from an identically-spelled Latin word. They have kept their “Latin” pronunciation, at least for the ending. This is not the way the Romans pronounced, or even a Late Latin pronunciation, but the way French scholars pronounced Latin from the Renaissance to the 20th century.

Any word that ends with the suffix -um and is either derived from Latin (even indirectly) or constructed in the same way is pronounced [-ɔm]. This includes a number of everyday words such as album, aquarium, minimum, maximum, etc. This applies even if the spelling of the word has been adapted, for example to add an acute accent on e when pronounced [e], as in référendum, sérum, … This also includes modern words constructed on the same model, such as the name of most chemical elements (hélium, uranium, röntgenium, …) and other words such as extrêmum, millénium, …

Even words that are not derived from Latin end but end in -um may be pronounced [-ɔm] by contagion, for example bégum (which French people know from the Jules Verne novel), rhum (pronounced just like Rome).

There are only a few exceptions. The main one is parfum: this word is derived from Latin, but -um is part of the root, not a suffix, so it’s pronounced normally, i.e. the m turns the u into a nasal: [paʁfœ̃]. Words that end in -oum are pronounced [-um]. The onomatopoeia hum denotes a sound that is closer to [hœm] or something like [xm], with a throat clearing followed by a long [m] without anything in between that French phonetics identifies as a vowel.

For more information about the history of the pronunciation of the final -um in French, see Pourquoi référendum ou péplum se terminent par /ɔm/ voir /œm/ et pas /um/ ?