I know this is a long shot but have you tried shooting canned air into the mic and speaker, or using a headset? Canned air has fixed two mystifying problems for us, and it seems equally likely that this random behavior is caused by hardware.
Sent from my Galaxy S4
Galaxy S Fascinate (Hellybean 4.2.2)(retired)
Galaxy Tab 2 10.1(Cyanogenmod)
Having trouble with the Fascinate? Check out Droidstyle's awesome guide here
Having trouble with the Galaxy Tab 2 10.1? Check out my guide here