Tone Sandhi: Sichuanese 四川话
From Chinese Wiki:


  连读变调现象在四川话口语中十分常见，但各地略有差异，以成渝片为例，大致来说四川话中的变调可以分为4类。
  一是重叠词中的变调，一般而言如果组成该重叠词的字声调为阳平或去声，则第二个字变调为阴平（例词：爸爸[pa2pa1]、婆婆[pʰo2pʰo1]、舅舅[tɕiəu4tɕiəu1]、帕帕[pʰa4pʰa1]）；同时，如果组成该重叠词的字声调为上声，第二个字变调为阳平（例词：姐姐[tɕiai3tɕiai2]、板板[pan3pan2]）。
  二是儿尾及儿化词中的变调，如果儿尾名词为阳平，儿尾变调为阴平（例词：娃儿[ua2ɚ1]、瓢儿[pʰiau2ɚ1]）；如果儿化词的最后一个字为阳平，该字变调为阴平（例词：葡萄儿[pʰu2tʰɚ1]、麻雀儿[ma2tɕʰyɚ1]）。
  三是特定字的变调，如“去”、“头”、“面”、“上”等虚词在多数情况下都变读为阴平（例词：下去[sic][saŋ4tɕʰe1]、屋头[vu2tʰəu1]、外面[uai4mian1]、身上[sen1saŋ1]）。
  四是二字词及三字词的第二个字在很多情况下变读为阴平（例词：姑娘[ku2niaŋ1]、经佑[tɕin1iəu1]、院坝[yuən4pa1]、龙门阵[noŋ2mən1tsən4]、明年子[min2nʲian1tsɿ3]）。


That is:


  The phenomenon of tones sandhi in Sichuan colloquial speech is very common, but every place has its own little differences, taking the Chengdu and Chongqing region as an example, Sichuanese tone sandhi can be approximately divided into 4 categories.
  One is the sandhi of overlapping1 words, generally speaking if the tone of the characters making up the word is a Yang Ping or a Qu, then the second one becomes a Yin Ping (examples: 爸爸 [pa2pa1]、婆婆 [pʰo2pʰo1]、舅舅 [tɕiəu4tɕiəu1]、帕帕 [pʰa4pʰa1]); at the same time, if the tone of those characters is a Shang, the second one becomes a Yang Ping (examples: 姐姐 [tɕiai3tɕiai2]、板板 [pan3pan2])
  The second category is the sandhi of words ending with 儿 and "erhua", if a noun ending in 儿 has a Yang Ping, the 儿 changes to a Yin Ping (examples: 娃儿 [ua2ɚ1]、瓢儿 [pʰiau2ɚ1]); if a word with erhua ends with a Yang Ping, that syllable changes to a Yin Ping (examples: 葡萄儿 [pʰu2tʰɚ1]、麻雀儿 [ma2tɕʰyɚ1]).
  The third category is the sandhi of specific characters, like “去”、“头”、“面”、“上” and other function words, which in many circumstances change to a Yin Ping (examples: 上去 [saŋ4tɕʰe1]、屋头 [vu2tʰəu1]、外面 [uai4mian1]、身上 [sen1saŋ1]).
  The fourth category is the second syllable of 2-character and 3-character words, which under many circumstances changes to a Yin Ping (examples: 姑娘 [ku2niaŋ1]、经佑 [tɕin1iəu1]、院坝 [yuən4pa1]、龙门阵 [noŋ2mən1tsən4]、明年子 [min2nʲian1tsɿ3]).


1This means words made with two identical characters.