the problems is not actually a bug or factory default but the way we remove the earphones from HTC after listening to music or Radio. We usually just pull it out.
Remember when you plug in you earphone you have to deactivate speakers for Earphone to work, (most especially for Radio). but when you just remove it without activating the internal speakers they remain deactivated.
I had this problem yesterday when my son pull off the earphone pin. I tried factory setting it did not work. but what I just did was to insert back earphone, de-active the earphone and my internal speakers just came back, loud and clear
one need to