PuTongHua or Guoyu, also called standard Mandarin, is one of the only 2 Chinese varieties standardized. Thus, some may think its pronounciation never changed. However, some sonud changes happened throughout years and were absorbed as new standard. Therfore, the standard pronounciation did change. Here I list some of them, some may be well known and some may be my own observations and is controversial, so correct me if I am wrong.
1. bo po mo fo > buo puo muo fuo
When ppl first learnt the pinyin, some may be confused abt the pinyin of bo po mo fo, at least I was. What I heard was absolutely buo puo muo fuo, but the pinyin was spelled as bo po mo fo. The reason is, when pinyin and standard Mandarin was first established, they did sound like how they are spelled, however, as o is a rounded lip vowel, it affected the consonants before it, forming a glide w between them, thus sounds like buo puo muo fuo, but the spelling remain the same.
2. j q x > z c s
This is an ongoing sound change. Originally, there were no j q x in Mandarin. When z c s are followed by i or ü, they were affected and palatalized to become j q x. On the other hand, g k h also became j q x when followed by i or ü. So j q x in modern standard Mandarin came from 2 sources, gkh and zcs before i or ü, they are in complimentary distribution, which means zcs and gkh will not exist before i and ü while jqx can only exist before i and ü. Thus, they have no function of differentiating meanings. Many cannot distinguish jqx and zcs(which is useless from the viewpoint of differentiating meanings), and pronounce jqx closer to zcs, shifting the pronounciation to what it was.
3. in-ing merger
This is a famous sound merger. The native Chinese dialects do not distinguish in and ing. When these native speakers speak standard Mandarin, they bring this feature into their accents, and even affect other accents.
4. w(/w/) > /ʋ/
Sorry for using IPA. But this is a very noticeable sound change. /ʋ/ is a labiodental approximant, which means you pronounce it by placing the lip and teeth to the position of pronouncing f sound but pronounce it like w sound. It is a free variation of /w/ and will not affect the meaning. This mostly occur in northern accent, but it is becoming a general trend.