As a non-English word, the pronunciation of the word "sandhi" is rather diverse among English speakers. According to Sanskrit phonology it can be . English pronunciations include (identical with "Sunday" for some British English speakers), /ˈsændi/ (identical with "sandy" for those speakers without the bad-lad split), and /ˈsɑːndi/.
While it may be extremely common in speech, sandhi (especially external) is typically ignored in spelling, as is the case in English, with the exception of the distinction between "a" and "an" (sandhi is, however, reflected in the writing system of Sanskrit). External sandhi effects can sometimes become morphologized (i.e. apply only in certain morphological and syntactic environments) and, over time, turn into consonant mutations.
Most tonal languages have tone sandhi, in which the tones of words alter according to pre-determined rules. For example: Mandarin has four tones: a high monotone, a rising tone, a falling-rising tone, and a falling tone. In the common greeting nǐ hǎo, both words in isolation would normally have the falling-rising tone. However, this is difficult to say, so the tone on nǐ is pronounced as ní (but still written nǐ in Hanyu Pinyin).