At all conceptual stages (Sindarin, Noldorin, Gnomish) the voiceless stops p, t, k became voiced stops medially (and finally) after vowels, a sound change that also took place in Welsh (WGHC/§104i). This became a key feature of Sindarin’s soft mutation system, whereby p, t, k mutated to b, d, g. This mutation dates back to Gnomish (PE11/7) and continues to be well attested in Tolkien’s later writings. Some representative examples from various time periods:

• ᴱ✶yai-ka > G. gaig “weapon, tool” (GL/43).
• ᴱ✶teutá > ᴱN. túd “thigh” (PE13/154).
• ON. kelepe > N. celeb “silver” (Ety/KYELEP).
• ✶aklariquā [> *aklarepa] > S. aglareb “full of glory” (PE17/24).

This sound change must have take place after voiced stops became spirants after vowels, because we see t > d but not t > d > dh [ð]. There is also evidence this sound change took place after short final vowels vanished, as suggested by David Salo (GS/§4.106, GS/§4.108):

• ✶nāba-grota > nǭv-ʒrot > (North) S. Novrod “Hollowbold” (WJ/389).

However, this sound change must have taken place before short vowels vanished before morpheme boundaries, since the second element of compounds consistently show the soft mutation of p, t, k to b, d, g; this ordering of sound changes was also suggested by David Salo (GS/§4.115):

• ✶Turukondō > S. Turgond (PE17/113).
• ON. andatektha > N. andeith “long-mark” (Ety/TEK).
• ᴹ✶minitunda > N. mindon “tower” (Ety/TUN).

There are a couple of peculiar Noldorin examples where this sound change does not take place, all involving the combination tl:

• ᴹ√TALÁT > N. atlanno “to slope, slant” (Ety/TALÁT).
• ᴹ√TALÁT > N. atlant “oblique, slanted” (Ety/TALÁT).
• ᴹ√TALÁT > atlaud > N. aclod “sloping, tilted” (Ety/TALÁT).

Absent further information, it’s hard to guess why internal soft mutation did not occur in these words. There are other examples where tl > dl: S. edlen(n) “exiled” < ✶etlendā (PE17/51), N. adleithian “*release” (EtyAC/LEK).

Conceptual Development: It seems Tolkien adopted this sound change from Welsh in the 1910s and retained it thereafter, with rare exceptions as noted above.

@@@ Inhibited by following h, e.g. Arothir (late compound): VT41/9

GS/§4.108 but not aclod @@@

HSG/§2.4