That words mean largely by convention is a well-established truism of language analysis, attenuated only by the knowledge that there are such phenomena as onomatopoeia, among a range of sound-sense symbolisms/parallelisms. A more indirect manifestation of the latter is contained in the final consonant d of the newish verb meld in the meaning ‘merge, blend; to combine or incorporate’, whose first attestation (according to various dictionaries) is dated to 1936. The original meaning was quite other, viz. ‘announce’, as in cards; also ‘make known (by speech), reveal, declare’, the etymology being Germanic (as e. g. in Old Frisian and Old English). The origin of the new verb is explained as a blend between melt and weld.
What is interesting in this process is the appearance of the sound d, evidently borrowed from weld. Why would this phonemically lax (erroneously characterized as “voiced,” which it is phonetically) stop lend itself to the new meaning of the verb, which can be generalized as ‘merging’, ‘fusing’, etc? The answer resides in the semiotic characterization of laxness in stops in languages, like English, which have distinctive protensity in their obstruent system (unlike languages like Russian, for instance, where voicing is distinctive rather than protensity). Thus d (the lax member of the opposition) is to t (the tense member) as unmarked to marked. Markedness, nota bene, is defined as the restriction of conceptual scope; hence the marked member is always relatively more restricted conceptually than its unmarked counterpart. That is exactly what we have in the new meaning of the verb meld, viz. unrestrictedness, here concretized to mean indistinctness, i. e., ‘merging’ or ‘fusing’. That is the raison d’être for the sound d in meld, of which it is the icon of the verb’s sense.