십만 is pronounced as [심만] by 비음화(nasalization).
비음화 (Nasalization) (표준발음법 제18항,19항)
*Some final consonants change to nasal consonants ㄴ,ㅁ,ㅇ when they come before initial consonants of ㄴ,ㅁ in next syllables.
<final consonants> <nasalized pronunciation>
ㄱ(ㄲ,ㅋ) -----------------> ㅇ
ㅂ(ㅃ,ㅍ) -----------------> ㅁ
And first consonant ㄹ changes to nasal ㄴ at the next position of final consonants ㅁ,ㅇ,ㄱ,ㅂ in the before syllables.
<Nasalization Examples 1>
속눈썹 → [송눈썹] (eyelashes)
막내 → [망내] (the youngest of the family)
한국말 → [한궁말] (Korean language)
국민 → [궁민] (people)
믿는다 → [민는다] (to believe)
-합니다 → [함니다] (-one of endings)
-입니다 → [임니다] (-one of endings)
종로 → [종노] (a area in Seoul)
3루수 → [삼누수] (third baseman)
섭리 → [섬니] (providence)
착란 → [창난] (derangement)
<Nasalization Examples 2>
※ Many koreans pronounce even English words in the way of nasalization.
nick name → [ning name] like 막내 → [망내]
big money → [bing money] like 국민 → [궁민]
good news → [goon news] like 믿는다 → [민는다]
Bob Marley → [Bom Marley] like 밥 먹어 → [밤 먹어]
Meg Ryan → [Meng Nyan] like 착란 → [창난]
If you are interested in Korean phonetic change rules, these videos will be helpful.
1. 대표받침화 (Major 7 final consonants rule) https://youtu.be/5lyK75jR20w
2. 비음화 (The nasalization) https://youtu.be/wJAtx7I6ZCU
3. 구개음화 (The palatalization) https://youtu.be/mNhe1wC4rww
4. 격음화 (The aspiration) https://www.youtube.com/watch?v=Zn7A4yQ-FN8&...
5. 경음화 (The tensification) https://youtu.be/5aoc9W3Zy1k
6. 음운첨가 (The phoneme addition) https://youtu.be/0Z3T79bH-0Q
7. 유음화 (The liquidization) https://youtu.be/ZzuFvvszabA
8. 연구개음화 (The velarization) https://youtu.be/8Yk70TFRJhw
9. '의‘의 발음 (The pronunciation of 의) https://youtu.be/2-LC4sOnzR0