...wut...mio your doing it all wrong..

Ritsu adding '-san' onto Mio's name bothers me a bit. They're very close friends and don't add an honorific when addressing eachother.

Artist said:
『澪～！ちゃんと聞いてんのかー？』

唯『ねーねー！澪ちゃん達が面白いことしてるよ～♪』◆梓『………" 面白い"…ですか…？よく分からないんですけど…』◆紬『ふふふ♪あれは夫婦にしか分からないことなのよ～♪』◆梓『………(夫婦)？』◆真相⇒ちゃんと聞いてるのに、何度も言われて腹が立った澪さん◆注※イヤホンを口に突っ込んだからって、声が聞こえるわけじゃないよ！(←知ってるよ！)

"Mio~! Are you listening?"

Yui: Hey! Mio-chan's doing something really interesting~♪
Azusa: ... What do you mean, "Interesting"? I don't really understand...
Tsumugi: Fufufu♪ You'll understand when you're married♪
Azusa: ... (Married?)

Truth -> Mio got annoyed that Ritsu repeatedly asks that even though she's listening
Warning *You won't be able to hear someone just because you insert headphones into their mouth! (<-I know that!)

seabook said:
Ritsu adding '-san' onto Mio's name bothers me a bit. They're very close friends and don't add an honorific when addressing eachother.

It's a "tone" thing. I'm not great at explaining, but Ritsu's being extra polite because of the situation (since she's at a disadvantage), hence the "-san".