I've noticed that after a period of time, all sounds except for video/music (things I click to play) can't be heard. Keyboard clicks, receiving messages, I guess the notification/system sounds, just stop. However, I don't know if my microphone stops working during this time as well.
Turning the phone off then on again restores them, then a day or so later, they disappear again. Frustrating, but expect it to be fixed with beta 3.