The crack was a known defect corrected in later devices, caused by interference. It is unlikely that it will result in broken solder joints-- those are normally tough enough to withstand a reasonable amount of stress.

That also applies to the usb connector on the N800s. That said, defects do occur. When I was overseeing final quality on N800s the field-failure rate (those units escaping the factory) was maybe 1%. I doubt it's over 3% at this point. Cold comfort to anyone who receives a defective device, I realize.

Damage to small device microphones *tends* to be caused by device drops from a significant height (say a meter or so). A not uncommon damage-related issue and perhaps the reason for the experience mentioned in the previous post. The N800 owner should be persistent; odds are he will get it repaired free.