Ah yes, the flexibility of Japanese sfx.

First panel zawa is the usual "indistinct murmur/crowd chatter" zawa.

Second panel is the "uneasy atmosphere" zawa made infamous by Akagi and Kaiji. Technically it's the same sound of people murmuring, but the context (tense situation, people acting nervous, mouths barely or not moving despite the sfx) causes it to take on a different vibe.

In other words... it's the "sound" of pregnant silence.