As I understand it, the initial m is silent, so no 'implicit vowel' is required, the 'mn' is pronounced like 'n', e.g. 'new-mon-ik'. The other word you mention

As I understand it, the initial m is silent, so no 'implicit vowel' is required, the 'mn' is pronounced like 'n', e.g. 'new-mon-ik'. The other word you mention (actually spelled 'chthonic') is pronounced with the initiall 'k' or 'ch' sound, like 'k-thonic'. If you search Google for the words, or look on Wiktionary, there should be a handy audio link with examples.