Headset connector :Connect Microphone and Headset to Android Tablet
The ASUS Transformer has a 3.5mm socket to allow a headset to be connected. Your bog standard headphones connects as usual . However the socket is the same hole that you would use to connect a microphone. So to connect both headphones and microphone, such as the two 3.5mm jacks found on some Skype headsets , you need an adaptor.
It took me a while to find one at reasonable price there are a couple a white connectors and one called the Headset buddy which are around £10 to £14(no buddy of mine) Be careful most of the adaptors available are designed to split the headphone output into two, and so allow two headphones to listen to the same music, this wont allow mike and headphone to work together.
I found this one on amazon costs around £5 delivered, called the Sandberg Headset convertor and does the job.