Not really that difficult.

Greek aspirated voiceless stops (ph, th, kh) and all voiced stops (b,d,g) in Late Antiquity became fricatives, a rather common sound change found all over the world. Unaspirated voiceless stops (p, t, k) usually didn't change, but in certain environments, such as after the nasal consonants m and n, they were phonetically voiced (think of the nb in Istanbul from is tin polin. Later the syllable final nasals were largely lost, and the digraphs mb and nd could be used to render foreign b and d.