I thought I'd broken the 3.5 mm Jack Socket last night. When I plugged in my headphones the sound stilled played though the speaker and the headphones icon was missing from the Status Bar. I tried another set of headphones in the hope that the headphones had broken and not the phone but no luck either. As a last resort I rebooted the phone and this fixed the problem so it appears that the headphone socket includes a software switch with a bug in the code. Hopefully the new firmware will fix this issue too.