I have a mobile phone (SE K750i) with a 3.5mm audio adapter to connect
to a 3.5mm audio output source, which enables me to redirect the
volume from the phone to headphones or external speakers. However,
when I do this, I lose the microphone function of my phone. Is there a
way I can split the audio output to connect an external microphone to
the phone?

While I accept that an audio out will normally not support a
microphone, my phone came with a standard headset consisting of
earphones and a built-in microphone, which plugs into the same socket
where the new audio adapter goes.

Simon wrote:
I have a mobile phone (SE K750i) with a 3.5mm audio adapter to connect
to a 3.5mm audio output source, which enables me to redirect the
volume from the phone to headphones or external speakers. However,
when I do this, I lose the microphone function of my phone. Is there a
way I can split the audio output to connect an external microphone to
the phone?

While I accept that an audio out will normally not support a
microphone, my phone came with a standard headset consisting of
earphones and a built-in microphone, which plugs into the same socket
where the new audio adapter goes.

You could hack the plug that goes into the phone to add a microphone
socket on pins 3 and 4:
http://pinouts.ru/CellularPhones-P-W...i_pinout.shtml
It probably just wants an electret capsule with no battery or resistor.

Probably easier to splash out £20 on a stereo hands-free set.
Some of them come with a 3.5mm socket so you can swap phones or
drive a separate amp.

--
Eiron.