In linguistics, prosody is concerned with those elements of speech that are not individual phonetic segments (vowels and consonants) but are properties of syllables and larger units of speech, including linguistic functions such as intonation, tone, stress, and rhythm. Such elements are known as suprasegmentals.
Prosody vs. Discrete Phonology
In learning Thai, or any other language, the point of prosody is that learning to say individual consonants and vowels is incomplete, and that syllables, words, and phrases have additional features of pronunciation which deviate from the simple blending of vowels and consonants.
Pure pronunciation of consonants and vowels as discrete units, even when blended together, cannot reproduce standard speech. After all, standard speech is primary and written speech broken down into discrete consonants and vowels must necessarily be an abstraction (as helpful as it may be).
Singing has a big impact on pronunciation, as words are often sung very differently than spoken, in terms of sound, tone, stress, and rhythm. Therefore a beginning learner would find it more difficult to understand Thai (recognize words) in song. This is not to mention that sung Thai is different from spoken Thai in word choice and composition. For intermediate and advanced Thai language learners, singing and songs can be useful in extending the boundaries of understanding, and producing Thai words and phrases.
ร และ ล - R and L
The most obvious change in pronunciation is the R and L, where in spoken Thai (except in formal settings), the R sound is replaced with the L sound. Of course, in written Thai this is not the case, and one learns from the Thai alphabet that ร เรือ (rau rua) and ล ลิง (lau ling) are different from each other, but in practice ร เรือ is pronounced like ล ลิง. Since they both are low class consonants with the same final consonant sound n, all the tone rules apply the same.
In addition, in consonant clusters with ล ลิง, such as กล้วย (gluay) banana, which is colloquially pronounced กัวย (guay). Consonant clusters are a complex feature of Thai and need to be dealt with comprehensively.
Any deviation from standard phonology can be attributed to prosody. There is of course a fine line between phonology-prosody, and so not all deviations are necessarily prosodic, but could be understood as conventional. Careful linguistic study demarcates these lines. For the Thai language learner, it is enough to be aware that the pronunciation rules of vowels and consonants are not enough, and the complex addition of more and more rules to account for exceptions increases the burden on the learner.
Of course children do not learn their native languages through rules and explanations but by listening, understanding, and practicing authentic speech. Adults would do well to try and embrace this repetition, rather than seeking cognitive explanations which cannot be put into practice with any degree of fluency and in fact impede progress to that goal.