It's longer than "u": Ů = Ú
The difference between those two is nowadays simply a matter of orthography. Ú is used at the beginning of a word/word stem, ů anywhere else.
And as for why we have two letters for the same sound: "ů" evolved over time from the former diphthong "uo": dóm > duom > dům. A heritage from the 14-15th centuries, so to speak.
It's an audio fault. Look at this link for better audio version https://upload.wikimedia.org/wikipedia/commons/4/4e/Cs-mal%C3%BD.ogg