In the Ilkorin phonetic developments described in The Etymologies, it appears that voiceless stops were voiced before nasals, as was also the rule in Old Noldorin. The clearest example of this change is ᴹ✶yatmā > Dor. adhum [aðum] (EtyAC/YAT). Here the [t] must have become [d], which then spirantalized to [ð]. It could not have remained [t], because that would have produced **adum: compare ᴹ✶atar > Ilk. adar (Ety/ATA).

A less obvious example is ᴹ✶takmā > Ilk. taum (Ety/TAK). Here it seems that the [k] became [g], which then vocalized to [u]. The evidence in this case (as well as similar developments for Ilk. saum) is not as strong, since this change could conceivably be explained by the rule that voiceless stops voiced after vowels.

There are no examples of this rule for [p] > [b] (then > [v]), but it seems likely the rule would apply in this case as well.