For example, if a lyric contains “that you”, it ends up like “thatchoo”. One example of this I can think of is in Karma by Taylor Swift (I know, I know, but it’s one of the most popular songs I listen to). The line where she sings “Karma’s a relaxing thought/Aren’t you envious that for you it’s not?” sounds like “arentchoo”. It doesn’t happen every time but it seems to happen unless you’re consciously making an effort to not make that sound. An example of this is in Love Story where she sings “That you were Romeo/You were throwing pebbles”, and it sounds like if you were just talking to someone and said “that” and “you” separately.
I’m just wondering if this happens in other languages with different combinations of sounds? It probably happens with other sound combinations in English too, but this is the easiest example to think of.
This is called connected speech, I think your specific examples would be assimilation where two sounds blend together. There are lots of other sub-topics of connected speech too. I’m sure this pops up in most other languages as well because if you natively speak a language, it’s likely that you’ll naturally find yourself connecting words and sounds. Great question, reading up about this was interesting.
Here’s a link to a site with more examples: https://gonaturalenglish.com/connected-speech-fast-native-english-pronunciation/
Spanish has a ton of these. Sometimes the rules of the language are changed to avoid them. For instance, you’d expect “the water” to use the feminine version of “the” as “agua” ends in “a” (la agua), but because it causes connected speech and makes the stressed syllable difficult to hear, the masculine version is used instead (el agua) to avoid that awkwardness.