I was wondering, if, when hangeul was first invented, Did any of the characters that survived until today have sounds that are not the same as the ones they have nowadays, or, Were they pronounced slightly different before?
Of course, every language undergo sound changes over time, and Korean was certainly not an exception. Let's go over each sounds and see how they changed.
The current mainstream vowel change hypothesis(established by Lee Ki-mun in the 1960's) states that two major vowel shift has occurred since the 13th century:
The first shift occurred between the 13th and the 14th century
| 13c - 14c | front | central | back | |-----------|-------|---------|-------| | high | ㅣ(i) | ㅜ(ɯ) | ㅗ(u) | | mid | ㅓ(e) | ㅡ(ə) | ㆍ(o) | | low | | ㅏ(a) | | || \/ | 15c | front | central | back | |-----------|-------|---------|-------| | high | ㅣ(i) | ㅡ(ɯ) | ㅜ(u) | | mid | | ㅓ(ə) | ㅗ(o) | | low | | ㅏ(a) | ㆍ(ʌ) |
Then in the second shift, many diphthongs and triphthongs that ended with /j/ monophthongized:
ㅐ(ㅏ+ㅣ), originally /aj/ monophthongized to /ɛ/, and is currently merging with /e/. This is the same process how the French word air is pronounced /ɛʁ/. Similarly, ㅒ(ㅑ+ㅣ) /jaj/ became /jɛ/, and is currently merging with /je/.
ㅙ(ㅗ+ㅏ+ㅣ) /waj/ became /wɛ/, and now merging with /we/.
ㅔ /əj/ became /e/. Likewise, ㅖ /jəj/ became /je/, and ㅞ /wəj/ became /we/.
ㅚ(ㅗ+ㅣ) /oj/ monophthongized to /ø/, and is now almost merged with /we/. ㅟ(ㅜ+ㅣ) /uj/ monophthongized to /y/, and then changed to /wi/ recently.
The only vowel with *+ㅣ construct that survived this change is ㅢ. This is why ㅢ is very unstable in modern Korean and is almost all of the time replaced by another vowel.
Now, for the consonants.
Contrary to Modern Korean, Middle Korean distinguished 8 consonants in the coda position. Modern Korean only allows 7, which are ㄱ /k̚/, ㄴ /n/, ㄷ /t̚/, ㄹ /l/, ㅁ /m/, ㅂ /p̚/, and ㅇ(ㆁ) /ŋ/. Middle Korean also allowed ㅅ /s/ in the final position. In other words, Middle Korean pronounced ㅅ differently to ㄷ in the final position, which in Modern Korean both are pronounced /t̚/. That's why many of the final /t̚/ is written with ㅅ instead of ㄷ. (Example: 못 not, in Middle Korean 몯)
ㄱ, ㄴ, ㅁ, ㅂ, ㅋ, ㅍ, ㅎ didn't change a lot since the 15th century.
ㄷ + /j/ underwent palatalization in the 17c, and became palatalized ㅈ. For example, 됴심 became 조심.
Middle Korean ㄹㅇ consonant continuum changed to ㄹㄹ.
ㅅ was /sʰ/ everywhere in Middle Korean, then palatalized to /ɕʰ/ in the 16c, and then returned to /sʰ/ except where followed by /j/, which remains as /ɕʰ/. That's why ㅅ has two sounds, e.x. 사 /sʰa/ and 샤 /ɕʰa/.
ㅈ(ㅊ) was /t͡s/(/t͡sʰ/) in Middle Korean, then palatalized to /t͡ɕ/(/t͡ɕʰ/), which remain today. Although I heard that very recently people started to pronounce ㅈ as /t͡s/ again(I don't have a source though).
I'll finish here. There's much more, consider this as "in a nutshell".
Yes! A good example is the vowel ㅐ(ae). As you can see it is a combination of ㅏ(a) and ㅣ(i). So its original sound was ㅏㅣ(ai). But now it is pronounced as ㅐ(ae).
Add 1: An article from the Joongang Ilbo, one of major Korean newspapers: http://mnews.joins.com/article/7185800
Add 2: The important part is "영상 속에 소개된 발음을 보면 당시엔 '나랏'을 '나라쓰'로 발음하고 'ㅐ'를 '아이'로 읽는 방식을 따른 것으로 돼 있다."