The first image is a SFX. It's not a whispering sound effect, there is a SFX for that it's like "koso" or "hiso", but the SFX is a hissing sound and it's occurring while Sai's cut is healing.
Sakura is shedding a tear because either she's nostalgic. Naruto and Sasuke are saving the world together. It could be that, and/or the fact that it's a really anxious moment. People can get tears from high tension and anxiety. Their combo is meant to be the final blow that finally ends the war. (She's not even participating in the attack, lols.) If there is a big twist and it turns out that having a restrengthened army has a benefit, then it'll make sense for Sakura to have yet again not get to fight alongside Naruto and Sasuke.
On the third image focus on this chapter is on Sasuke, so I think she is thinking the same thing that Sai is, and they're serious because Sasuke is a missing-nin, a rogue, an enigma, achieves his goals ruthlessly, so it's natural to wonder what he's up to. They have a really really good reason to be concerned.