There are 2 aspects for initial consonant clusters to affect the tone birth and development. On the one hand, initial consonant clusters cause rising tone; on the other hand, they plausibly cause falling tone. This study attested these controversial issues in Mon of Mon-Khmer language family and in Thai of Tai language family. The informants were divided into 3 age groups; over 60 years, 35-45 years old, and below 20 years. Acoustic analysis and auditory test were conducted using Praat. Statistical analysis, e.g. t-test and coefficient r, was also used. The effect of initial consonant clusters on fundamental frequency of semitones was analyzed. It was found that, in the 3 age groups of Mon, semitone values of the vowels following single initial consonant were insignificantly higher than those following its pairs which was initial consonant clusters. However, semitone values of the vowels following single initial consonant were not always higher than those following its pairs which was initial consonant clusters in Thai. Regarding pitch contour, auditory test revealed that initial consonant clusters cause either rising or falling contour; both aspects of initial consonant clusters effect towards the contours are correct. This study also pointed sound change patterns in initial consonants of Thai, but not in Mon as Mon speakers strongly preserve initial consonant clusters. The causes of language change were studied through reflection. There were 2 factors causing language change; internal factors (e.g. social values, speech organs) and external factors (e.g. media and technology, foreign language). In Mon, external factors were the main factors causing the change, but in Thai both internal and external factors were the main causes. This can be interpreted that Mon people have a tendency to preserve their language much more than the Thai. One significant factor for language change in Mon was political and war.