For both Sindarin and Noldorin, in compounds where a voiced dental spirant [ð] came in contact with a nasal after various vowel losses, the [ð] vanished, as noted by David Salo (GS/§4.119). There are several examples in both Sindarin and Noldorin:

• Kh. kheled + S. morn > S. Hele(ð)vorn “Black Glass” (RS/466; PE17/37).
• ✶Gala(da)ndil [> *geleðndil] > S. Gelennil “Lover of Trees”, with variant Gleðennil (PE21/83).
• N. Golodh + N. mîr > N. Golo(ð)vir “Silmaril, *(lit.) Noldo-jewel” (Ety/MIR, ÑGOLOD).
• ᴹ✶Eledandore > Eleðndor > N. Elennor “Land of the Elves” (Ety/ELED).

The last example is the clearest demonstration of this change. The examples Hele(ð)vorn and Golo(ð)vir seem to hint that the loss of [ð] occurred after the change of m > v, but I think this was simply notational convenience on Tolkien’s part, indicating there used to be a [ð] that was lost earlier.

Conceptual Developments: The examples G. bridh(n)ir “queen” (GL/24) and G. goredhnir “cousin (f.)” (GL/41) indicates this change did not occur in Gnomish. There aren’t enough examples in the Early Noldorin of 1920s to figure out whether or not this sound change occurred at that conceptual stage.