I presume that you computer is a laptop with a built in microphone.
Look in the Windows Sound Control Panel to see what recording device is set as the default. Set that to the input that you want to use.
Then open (or restart) Audacity.
The “mapper” input in the Audacity device toolbar selects whatever is the default in the Windows Sound Control Panel.
You can either, use “mapper” (assuming that is set in the Windows Sound Control Panel to the input that you want to use, or explicitly select the input that you want to use.
Note: If you have Skype or other “voip” application, ensure that is completely shut down before you start. Skype is known to “hijack” the sound system (which it needs to do to be able to work properly).
When you’re in the “Recording” tab of the Windows Sound Control Panel, you should see a green meter jiggling up and down when the sound card is receiving sound. Is that happening? If so, is it responding to your “vinyl / cassette sources through the OUT option on my amp” or to the laptop microphone?
@Steve - It does jiggle [love the description] but will only recognise the external sound, whether the cable is in or not, therefore all I hear when I attempt to record into the microphone socket, is myself speaking/ clapping etc
@Gale - Thank you sir but sadly the external mic is already set as the default recording device and it makes no difference.
I’ve also double checked by attempting different sources going into the laptop, but no difference.
It seems that your computer may no longer recognising that you have something plugged into the socket.
Carefully examine the cable, and in particular, ensure that it is plugged into the correct hole on the laptop (I know that sounds obvious, but these things do happen ) and check carefully, when plugging into the laptop, that it “feels” right (the sockets can sometimes become detached from the circuit board inside the computer).
Ah ha, now that is strange - I don’t think that is likely to be due to a hardware fault.
Do you know how to “roll back drivers”? If you do, see if the sound card drivers can be rolled back to an earlier version (Windows may have updated the drivers for you and caused the current problems)