What might explain this phonetic change, in Cantonese and Mandarin?
Please educate me if there's a more fitting term than 'phonetic etymon'.
Interestingly, there is another character 褢 [same reading as 褱], with 鬼 inside.
Here are a few parts where Humanum mentions the relationship between 褱/褢/鬼 and even 歸:
So there is a usage of 褱 that means 歸 [kjuj] "give" and related to 饋 [grjujs] "make offering to."
Here's evidence that 褱 [gruj] and 鬼 [kjujʔ] were homophones. Actually, 歸 and 鬼 might be cognates (人所歸爲鬼), but I digress. The point is, [kjuj] is the much more likely target reading to hit for 褱, not the [k-l- | -p] initial/final pattern of 眔.
褢 was used in the Han-Tang era to mean "hold in," without the emotional semantics.
又表示安撫、招來，毛公鼎：「率褱(懷)不廷方」，意謂安撫不來朝覲的方國。陸賈《新語．道基》：「附遠寧近，懷來萬邦。」 ... 又借「鬼」為「懷」，表示安撫，《睡虎地秦簡．為吏之道》簡46-47貳：「君鬼(懷)臣忠，父茲(慈)子孝，政之本殹(也)。」
This is the same as the "give" meaning.
We learn several things:
- 鬼/褢/歸/褱/懷 were near homophones and were interchanged in writing at various times.
- There is a "hold in" meaning of 褱 that seems cognate with the "hide tears/miss" meaning for which the character was apparently constructed, possibly a direct extension, but more likely to be the actual root -- there is a Proto-Tibeto-Burman etymon [*kway] that means conceal/hide.
- There is a (fairly archaic) "give" meaning of 褱 that is related to some of the meanings of 歸 (namely its use as 饋), and seems like a phonetic loan and unrelated to the first meaning.
- The reading of 褱 [gruj] is fairly secure given the readings of the other characters, in particular, the [g/k-r/j-uj] pattern. It is most likely not related to the reading of any crying related etymon, for which there is a fairly established Proto-Sino-Tibetan reconstruction [*krap], nor to the 泣 [khrjəp] that is posited to be 眔, nor to the 徒合切 [dup] spelling given by Shuowen for 眔, nor to 涙 [c-rjuts], 隶 [c-rəts], or 逮 [ləts] that have been written with 眔 in Oracle Bone for its meaning and/or sound.
- See also my comment to droooze's answer on why I believe 褱 is a rebus.
So, to answer the question: "Why doesn't 褱 sound like its phonetic etymon 眔?" It's because it has no sound component, and 眔 is not its phonetic part. On the other hand, the later character 褢 would be good candidate if you're looking for a legitimate phonetic part for a cognate of 褱.
The more common term would be phonetic component or sound component.
Anyway, if Zhengzhang Old Chinese constructions are reliable,「眔」being the phonetic component of「褱」can possibly be seen:
- 「褱」, /*ɡruːl/ > /ɦˠuɛi/ > huái (Pinyin), waai4 (Jyutping)
- 「眔」, /*l'uːb/ > /dʌp̚/ > dà (Pinyin), daap6 (Jyutping)
Bolded parts are my own suggestion of the phonetic correspondence.
Note,「眔」is very commonly interpreted by scholars as a picture of tears「氺」(water) flowing down an eye「目」.「褱」, by most standard dictionaries, is thought to be the original character of「懷」, with the following meanings:
- 懷藏/包圍/懷孕 (to wrap/conceal/be pregnant)
- 思念；關心 (to miss/care about somebody)
Supposedly, there is a semantic connection between「眔」and「褱」in the second definition.「眔」is also thought to be the original representation of the word now written as「涕」.