If you thought this mountain's name was pronounced 'Caradras', you're in good company - even the BBC got this pronunciation wrong in their classic radio adaptation of The Lord of the Rings.
The confusion arises because modern English uses the combination 'th' to refer to two different sounds: the 'voiceless' sound in 'both', and the 'voiced' sound in 'bother'. In Elvish, these two sounds were represented by different characters - typically thúle () and anto () respectively. Tolkien adopts a consistent transliteration scheme for these Elvish characters: he uses 'th' to refer exclusively to the voiceless sound, and 'dh' to the voiced.
'Caradhras' is probably the most prominent use of this 'dh' sound, but it does occur fairly frequently in other names: Aredhel, Maedhros and Caras Galadhon are three important examples.