No, adjectives that precede the noun cause soft mutation. In this case we have a discussion beneath in Welsh which explains why this hasn't happened here. The reason why is because it is not an adjectival construction but a genitive one where "De" belongs to "America" i.e a more literal translation would be "The south of America", but in English we'd just tend to say "South America.
Isn't it like 'het Owen'? In other words, 'America' here is a 'proper noun', the name of a country - just as you'd name a person.
I find substitution a really helpful grammatical tool, and so far 'het Owen' has been very useful to my stumbling efforts to get to grips with how Welsh works in these cases (plus lots of help from moderators and other contributors - diolch!).