سلام
/ˌem.piːˈθriː/ us
/ˌem.piːˈθriː/
an MP3 file/download/device
MP3 technology/format
The answer has nothing to do with music as you have mentioned in the tags, it’s all about the English language. When we pronounce the letter ‘M’ as in mp3(as empee3) we first pronounce the letter ‘E’ , which is a vowel and we always use ‘an’ and not ‘a’ before pronouncing a vowel. Hence, we use ‘an’ before pronouncing the letter ‘M’ coz of that ‘e’ sound in the beginning.
E.x. “That person is an MP” is correct but “that person is a MP” is wrong.
This was just another otiose exception in English language
صدا/تلفظ اون حرف مهمه نه خود اون حرف
مشابه hour که به صورت an hour
و university که a university نوشته می شه
دیدن این ویدیو هم کمک می کنه به فهم بیشتر (در لینک زیر یا اپ زبانشناس/انگلیسی با پاپا/ گرامر)