Answer to: device will no longer accept headphone jackThere is no need to replace the entire device,. Usually this happens when a fair amount of dirt/lint accumulates in the socket due to it's magnetic properties. Try cleaning it with an air can, hot air blow or a cotton swab. If this doesn't work try a second pair of headphones to ensure that there is no problem with the headphones. If the problem still persists then you can go to the local Apple store or repair shop and get the socket of your IPod replaced.
Answer to: How do I change the Mic/reciever for the P11 EarPhone?Connect the USB headset to a PC and record your sound using an application such as sound recorder and replay it to ensure that the problem is with your mic and not their speakers or volume control. If yes then check the connection between the mic boom and the headset. A broken connection might result in a faulty microphone. Check the troubleshooting and repair pages on how to replace the microphone.