While the audience’s reaction is not visible in the videos, footage shows the servicewoman and men standing behind Biden, as well as his wife Jill Biden, smiling throughout his remarks.
Here are their reactions, just after: https://youtu.be/2FUcxqGObZc?t=377 All smiles.
Found the context.
It's during the Senate Ceremonial Swearing-in on January 6, 2015. See 1:21:15 in this video: https://www.c-span.org/video/?323601-1/senate-ceremonial-swearing-vice-president-biden
Or clipped for your convenience here: https://www.c-span.org/video/?c4860943/
I think the older girl is just laughing at Biden's joke about the brother (out of frame here) keeping the boys away.
Man down: