Schuessler has early meanings for 皆 (and 喈): “... ‘Be together with, agree , all’ [Shi]; ‘be or do in unison’ [Shi]”. Subsequently he puts forward as etymologically related 偕 and 諧: “偕 ...‘Together’ [Shi]” and “諧 ...‘Be concordant, harmonious’ [Shi].⁴
I take this to mean that the graph 諧 was created for an extended or special meaning of 皆/喈, which would explain why 言/speak was added as signific.
The original graph 皆 consists of two people accompanying or following each other, and an element that has not been identified with complete certainty, though many scholars seem to think that it is a corruption of 曰/say.⁵ This element was already at the seal characters stage standardized as 白 (strictly meaning “white” but really an empty element that does not contribute meaning).
The top element 比 shares its origin with 从 (modern Japanese 従). At the earliest stage (oracle bone inscriptions) it was used for “accompany” or “follow,”⁶ which seems to lead up nicely to Schuessler’s meanings for 皆, “be together with,” “in unison” etc. The subsequent “be concordant, harmonious” covered by 諧 is not far from that either. If 白 really was 曰/say then that would have added a verbal aspect to 皆, as does 言/speak to 諧 (speak/sing in unison/harmony).
In making a mnemonic one can go several directions. One is for example to take 皆/everybody as a known, indivisible element. Another way is for example analyzing 皆 as 比/compare and 白/white.
Mnemonic: Speech can be like white noise, which is comparatively harmonious.
1. The part of the graph that conveys meaning and is specifically added to classify or determine the meaning of the graph as a whole.
2. A phonetic or phonetic complement gives a hint for the sound of the word the graph stands for. An example in English would be nd in the writing 2nd.
3. Since Xǔ Shèn 許慎 the technical term for this is 亦聲 yìshēng (亦声 ekisei in Japanese). Graphs within this class usually come into being when, for clarification of the intended meaning, a signific is added to a graph that was already in use.
4. Schuessler 2007, pp. 310-311. He is not sure whether 階 points to the same word as 皆/喈. As for other graphs that use 皆 as phonetic, he treats as unrelated 稭/秸, doesn’t mention 堦 (however, that graph seems to point to the same word as 階 anyway), and doesn’t mention 楷, 鶛, 湝, 蝔, 鍇, 鍇, 揩, 瑎 or 龤. As for why 皆 and 喈 where originally used for the same word, but now seem to point to different words, he doesn’t explain that either.
[Shi] refers to the Shījīng 詩經 (ca. 1050-600 BCE).
An example from the Book of Rites for 諧, translation James Legge:
The superior man observes these rules of propriety, so that all in a wider circle are harmonious with him, and those in his narrower circle have no dissatisfactions with him. Men acknowledge and are affected by his goodness, and spirits enjoy his virtue.
5. SH: 1099.
6. Ochiai 2016, pp. 23-24.