I think it's a mixture of disbelief, shock, embarrasment and jealousy. They are trying to understand what happened infront of them and how to react to that - other than simply mounting him.
You have to keep in mind that these girls just witnessed something utterly unreal. A boy giving a compliment to a girl is already rare in itself - it might happen, but none of them ever experienced that. Plus the girl was Miku. Now think about it... normally, if Miku just talks to a boy, he will start to cry, shiver, get angry or run away in panic... Miku is seen as unpopular from a boys perspective and she instills fear. This time it's a complete reversal. Miku ends up with teary eyes (in the end it's out of joy), she's the one who runs away and she is almost rendered speechless. What Sou did there is so far off the charts that anyone who watched the scene will question their own sanity. These two probably think something like this:
"Awawa, what happened just now? OMG! OMG, OMG... I can't believe it... the fuck did I see? This was no hallucination, right? It really happened... this boy looks at Miku-sama like that? Miku who makes boys cry is in tears? Sou told her that he likes her big breasts? That's gotta be a lie... but... but, when I looked at him, it felt like he was serious. A man likes Miku? I mean, he confessed to her right infront of us! Damn, Miku, I'm happy for you!! But I'm so jealous... should have been me!! Sou, please, also look at me and... but would he do that? I-If it's him, he might... but, calm down! Don't jump to conclusions yet, girl! Do I even have a chance against the queen? What to do... Miku is away right now... should I wait? No NO!! I need to do something... anything! Think! This might be my only chance to score a boyfriend! Should I go on attack? But he might hate me, that would be the end of me. What is with my friend? Ugh... she looks just as troubled. If I don't act now, I will loose. Either the one right next to me will make her move or Miku comes back and then it's too late! I need to know if I heard him right! But how should I do it? What if I just ask him to make sure? Ahh Ahh, but will he actually answer... damn, screw it, girl!! I'm gonna ask!"
EDIT: typos