I'm asking this question as a non-linguist native Korean speaker.
I will use the word "fortition" as a translation of 경음화 or 된소리 되기.
The Standard Korean (표준어) does define some complicated rules for the fortition of Korean consonants, but this is not complete enough and never will be to describe how the words are actually spoken in everyday speech. And I personally disagree with many suggestions that the standard makes. The list below explains enough about the irregularity.
(word : standard-suggested pronunciation : actual pronunciation)
(long and short vowels are not distinguished here because they are not distinguished, at least among the people I hear speaking every day)
- 효과 : [효과] : [효꽈], [효과] (hyo-gwa : hyo-kkwa, hyo-gwa)
- 과대표 : [과대표] : [꽈대표] (gwa-dae-pyo : gwa-dae-pyo : kkwa-dae-pyo)
- 사건 : [사껀] : [사껀] (sa-geon : sa-kkeon : sa-kkeon)
- 강가 : [강까] : [강까] (gang-ga : gang-kka : gang-kka)
- 관건 : [관건] : [관껀], [관건] (gwan-geon : gwan-geon : gwan-kkeon, gwan-geon)
- 불법 : [불법] : [불뻡], [불법] (bul-beop : bul-beop : bul-ppeop, bul-beop)
- 조금 : [조금] : [쪼금], [조금] (jo-keum : jo-keum : jjo-keum, jo-keum)
- 들고양이 : [들꼬양이] : [들꼬양이] (deul-go-yang-i : deul-kko-yang-i : deul-kko-yang-i)
- 실험적 : [실험적] : [실험쩍], [실험적] (sil-heom-jeok : sil-heom-jeok : sil-heom-jjeok, sil-heom-jeok)
- 시드니 : [시드니] : [씨드니] (si-deu-ni : si-deu-ni: ssi-deu-ni) "Sydney"
- 서비스 : [써비쓰] : [써비쓰] (seo-bi-seu : sseo-bi-sseu : sseo-bi-sseu) "service"
- 창고 : [창고] : [창꼬], [창고] (jang-go : jang-go: jang-kko, jang-go)
I also want to say about ㅅ insertion (사이시옷), but that will make this post too long.
As you can see in the examples, the problems mostly occur in loanwords. This is because, unlike pure Korean words, in the case of which almost always everything is written as it is spoken, loanwords have its own standard rules of writing them in Hangul depending on whether it is Sino-Korean, English, French et cetera. But the words written following these set of rules often does not match how it actually is spoken among the people. For example, the French city Paris is written as 파리 according to the standard, but it is more often pronounced [빠리], not [파리]. Is the problem just as simple as "most of the people speak in this way for this word, and in that way for that word", like how the modern English orthography works? Or are there some trends or patterns in this irregular fortition phenomenon of the Korean language, that seems irregular but not quite in detail? Can you explain me a bit or cite some related research on this topic?