According to dictionaries, the correct pronunciation of "馄饨" is hún tún, but I have never heard any native speakers pronounce the word this way; instead, almost all native speakers say hún dùn. Why?
Because native speakers don't actually know the tone without being taught it.
Northern Mandarin pronounces it as
húndun, with the second syllable having the light tone (轻声). The morpheme 饨 is not used anywhere else, so it only appears in native speech without a tone.
It's not rare for native speakers to reinterpret this syllable as "originally having the 4th tone" because of the acoustic similarity when pronouncing it with the light tone (there's a slight phonetic fall particularly as it follows a 2nd tone syllable). So when they try to shake off the light tone accent which is associated with being regional vernacular, they arrive at
d is unusual. It may be a sporadic change. The only other word I can think of, where the initial (声母) may be lenited in a light tone, is 糊涂
húdu. Many people do "correct" the pronunciation incorrectly to
húdù, but this problem here is less severe because, unlike 饨, 涂 is widely used elsewhere with the known pronunciation of
tú. Hence when speakers (especially literate ones) try to enunciate it, they arrive at
馄 (hún) in '馄饨'(hún tún) is pronounced correctly
Since 饨 (tún) looked like 沌 (dùn), people may have just confused the two characters.
Cantonese write 餛飩 as 雲吞 with the same pronunciation
雲吞 /wan4 tan1/ (Cantonese) / hun2 tun1/ (Mandarin)
餛飩 /wan4 tan1/ (Cantonese) / hun2 tun1/ (Mandarin)
Which one comes first: the pronunciation or the regulation of it?
You can only say one pronunciation obeys a regulation or not, but you cannot say which pronunciation is "wrong". If in one area, 99% of people pronounce it in the same "wrong" way, you cannot say it is wrong, you can only say, the pronunciation of the word in that area is different from what is regulated by a central government. For generations, people from my home town ShanDong call such a thing hun2dun4, why someone from another area has the authority come and blame that "you are all calling it 'wrong'."?
I agree that regulation is important as the standard for formal usages. For example, in the College Entrance Test, all the pronunciations have to be the same as the latest version of 《现代汉语词典》 (Modern Chinese Dictionary) by 商务印书馆. But it is not the case in daily life. One needs to respect other peoples' life. If you go to a restaurant in Shandong and order “hun2tun2”, people there will probably be confused (or amused). Same thing, If I, as a ShanDongness, go to SiChuan and want to order the same thing, I need to call it 抄手(chao1shou3) to be understood, and also to show my respect to people who live there.
PS 1: I don't think it is an issue of dialect, it is a regional variation of only one word. I think it is the same kind of variation of 菠萝 vs 凤梨 and rubber vs eraser. ShanDongnesses who speak Mandarin instead of ShanDong dialect will also call it hun2dun4.
PS 2: There is a possibility that regulation is changing people's life gradually. There might be a day that more people in ShanDong call it hun2tun2 because more people are willing to obey the national regulation in daily life. But even with this said, I don't think it will happen in my generation.
PS 3: Governmental Dictionaries, 《现代汉语词典》 and 《新华字典》 especially, are changing over the years. For example, the word 说服 was originally regulated to be pronounced as shui4fu2, but because too many people are pronouncing it as shuo1fu2, in the revision of the dictionaries, the pronunciation is directly changed to shuo1fu2.
Let's check comprehensive source like zdic.net for 馄饨.
You will notice hún dun ㄏㄨㄣˊ ˙ㄉㄨㄣ come with 国语辞典 tag. This mean the pronunciation is generally used in Taiwan.
While hún tún is used by mandarin speaker in China.
In fact, you can call this "mandarin dialect", though some Taiwanese are not happy with it if you say so.