啊 changes pronunciation based on the preceding sound: after -n it sounds like 'na', after -ng like 'nga', after vowels like 'ya' or 'wa'. In writing it's always 啊. Some texts write the assimilated form (呀 / 哇 / 哪) to match speech.
usage
Toneless 啊 softens a sentence - flat statements become friendly remarks, sharp questions become curious ones, blunt commands become nudges. The four toned interjection readings (啊 / 啊 / 啊 / 啊) are a separate use - those stand alone at the start of a sentence as emotional cries.
Left-side mouth radical 口. Sound-words and exclamations almost always carry 口 - the radical signals 'this is something coming out of a mouth, not a lexical word'. Same role in 呀, 哦, 哇, 呢, 吗.
Right side 阿 supplies the sound. 阿 is itself a phono-semantic compound (阝 + 可) and a common name-prefix in southern China (阿明, 阿姨). Here it functions purely as the sound, with 口 doing the 'this is an exclamation' work.
