The Thai language was devised to serve two main purposes: to write Thai words and to write Sanskrit (or Pali) words. For this reason, the Thai alphabet has one consonant symbol for each Sanskrit sound and several additional consonant symbols which, presumably, corresponded to Thai sounds which did not occur in Sanskrit.
The Thai consonant symbols which corresponded to Sanskrit consonant sounds were as follows:
ก (k), ข (kh), ค (g), ฆ(gh), ง (ng),
จ (c), ฉ (ch), ช (j), ฌ (jh), ญ (ny),
ฏ (T), ฐ (TH), ฑ (D), ฒ (DH), ณ (N),
ต (t), ถ (th), ท (d), ธ (dh), น (n),
ป (p), ผ (ph), พ (b), ภ (bh), ม (m),
ย (y), ร (r), ล (l), ว (w),
ศ (sh), ษ (S), ส (s), ห (h), ฬ (L),
The remaining consonant symbols in the Thai alphabet are as follows:
ฃ, ฅ, ซ, ฎ, ด, บ, ฝ, ฟ, อ, ฮ.
These remaining symbols presumably corresponded to consonant sounds which existed in the Thai language which did not occur in Sanskrit. My best guesses for these sounds (based on what I have read as well as the position in the alphabet where these symbols occur) is as follows:
ฃ, like ch, as in Scottish loch,
ฅ, like r, as in French rouge,
ซ, like s, as in English measure,
ฎ, possibly a glottal stop followed by a retroflex T or D,
ด, a glottal stop followed by a t or d,
บ, a glottal stop followed by a p or b,
ฝ, like f, as in English fan,
ฟ, like v, as in English van,
อ, (when used as a consonant) is (and probably always was) a glottal stop,
ฮ, I think this letter was added to the alphabet later, sounding like the existing letter ห (h), but producing different tones.
I guess that all of the symbols which corresponded to Sanskrit sounds would have originally made the same sound whenever that sound actually occurred in the spoken Thai language. So for example, I guess (from what I have read) that the Thai letters ค, ช, ท and พ were originally pronounced as the unaspirated, voiced Sanskrit sounds g, j, d and b (although they are now pronounced like the aspirated, unvoiced Sanskrit sounds kh, ch, th and ph).
I have several questions
Is it known whether the spoken Thai language(s) ever used the voiced, aspirated Sanskrit consonant sounds gh, jh, dh, bh (which do not occur in modern Thai, or in English)?
Is it known whether the spoken Thai language(s) ever used the retroflex Sanskrit consonants T, TH, D, DH, N (which do not occur in modern Thai, or in English)? My only (somewhat feeble) reason for guessing that these sounds may have occurred is the existence of the Thai consonant symbol ฎ, which may have represented a glottal stop followed by a retroflex T or D. If these retroflex sounds did not occur in Thai then what was the sound produced by the consonant symbol ฎ?
Are any of my above guesses, as to the original pronunciations of the symbols ฃ, ฅ, ซ, ฎ, ด, บ, ฝ, ฟ, อ, ฮ (which do not correspond to Sanskrit consonants) known either to be true or to be false?
Finally, if the answers to some of my questions are known, or at least if there are some conjectures as to the answers, then what is the basis on which the conjectures are founded? That is, what is the evidence that might lead one to make more educated guesses than I myself have made (I am only hoping for a brief answer here)?