In Noldorin and Gnomish there are a number of examples of initial mr- becoming br-; this sound change must have occurred after unstressed initial syllables reduced to favored clusters. This same sound change occurs in the history of Welsh, sometime between Proto-Keltic and Britannic (WGHC/§99ii). Some Gnomish and Noldorin examples:

• ᴱ✶mṛt > G. Bridwen “Fate” (GL/24).
• ᴹ✶morókō [> *mroko] > N. brôg “bear” vs. ᴹQ. morko (Ety/MORÓK).
• ᴹ✶mrekjā > N. breig “wild” vs. ᴹQ. merka (Ety/MERÉK) [these forms were later deleted and replaced by derivatives of ᴹ√BEREK].

In Welsh there was a related sound change where ml- became bl- (WGHC/§99ii) and there are examples of this phonetic development as well in Gnomish and Early Noldorin (but no examples after the 1920s):

• ᴱ✶mol- > G. bloss “wheat” (GL/24).
• ᴱ✶mḷgo > ᴱN. bliw “oil” (PE13/139).

In Early Noldorin and Gnomish, there are further examples of dental nasal [n] becoming [d] before [r], [l]:

• ᴱ✶nṛþ- > G. drith “savour” (GL/30).
• ᴱ✶norokā́ > G. drog “slave” (GL/31).
• ᴱ✶nṛga > ᴱN. dre “wearisome” (PE13/150).
• ᴱ✶nḷđ [> *dlith] > G. glith “dale, hillslope” (GL/40).

For the last example, bear in mind that [dl] became [gl] in Gnomish, as described by Roman Rausch in his Historical Phonology of Goldogrin (HSG/§2.4). He also identified the above sound changes in Gnomish, where initial nasals [n], [m] becoming stops before [r], [l] (HSG/§2.1).

Neo-Sindarin: There is no evidence of any of the above changes in Sindarin in the 1950s and 60s. Since mr-/ml- > br-/bl- in Welsh, Tolkien might have allowed these changes in Sindarin as well, and I include them in my phonetic rules for Neo-Sindarin. The nr-/nl- > dr-/dl- changes seem more dubious to me, but still possible. David Salo attributed the mr- > br- change (but excluded the ml- change) to Old Sindarin in Gateway to Sindarin (GS/§4.36). Salo also included ŋl- > gl-, but I can find no evidence of this and I don’t know where he got it from.

HPG/§2.1