Did you know that the pronunciation of 普通話, Putonghua or the modern standard Mandarin Chinese, dates back only about 400 years?

From 中国华文教育馆 (PRC’s Oversea Chinese Language and Cultural Education Online), the Manchurian people, after establishing the Ching Dynasty and succeeding the Ming Dynasty in the 17th century, had to learn one of the northern Han dialects for ease of communication.  However, they couldn’t learn it well and eventually developed a “Manchudized” version, which eventually turn into the pronunciation we know today.

“与南京官话保留了中原古音不同，王照提倡的北京官话受北方游牧民族特别是满族的影响很大。满洲入关以后，满语完全不能适应新的生活需要，不得不学习和借鉴汉语。但是，满族人不能区分尖团音，也不会发入声，因此，满洲贵族所说的北京官话是满族化了的汉语，也被戏称为“五音不全”的汉话。”

From 百度：

“南北朝时期开始，中原雅音南移，作为中国官方语言的官雅言逐渐分为南北两支。六朝即南京话为汉语标准语，明朝永乐年间建都北京时从南京北调40万人口，超过北平原有人口。清入主中国，旧北平话逐渐演变形成了北京话。清雍正六年设“正音书馆”，以北京官话为标准语，在全国推行，以后北京官话逐渐取代南京官话成为中国官场主流的标准语，有人也称之为北方官话，和被称为南方官话的南京官话相对应。清末进行国语编审，民国初年拟定国音，“京国之争”以后实行以北京官话为基础的新国音，自此以北方官话为蓝本的国语（普通话）成为中国官方标准语言。随著现代教育、传媒的普及发达，当代的北京官话 – 普通话，在华语圈有向各种方言渗透的趋势。”

Therefore, the venerated Tang poems dating back to the 7-10th century and classics before Ching Dynasty were not written with such pronunciation in mind.

Below are a couple of interesting video clips: