Same sound problem
I have a similar problem.
I have the latest available android installed on the Iconia a100 (Acer_AV041_A100?1.037.00_WW_GEN1, ICS 4.0.3)
and I had no problem with the headphone sound until last week. I have used it regularly, almost every other day.
Since then headphone icon does not show up when I connect it to the device. Sound is correct via the speakers but it is not an option on public transport.
I have tried several headphones, to switch the device on/off, to increase/decrease volume, to switch on with headphone plugged in but nothing happens.
I have checked the plug as far as I could: there is no dust or anything unusual in the hole.
I could continue with a factory reset (after archiving everything) but I am not very optimistic about that.
Is factory reset could help? Is it a hardware issue or am I missing something?