No audio output when using external mic - iMac

I have an iMac (c.2016).

I want to be able to improve audio output when using FaceTime by using an external mic.

I have both a stand alone mic and a combined headphone and mic headset. The former has one headphone type jack which has only one black ring (mono?). The latter has both a red and black jack with two black rings (stereo?).

If I try using the stand alone mic via the iMac's headphone socket, the mic works but it seems to cut out the sound from the built in speakers so I can't hear the other party(s). The same happens if i use either of the jacks in the headset. I assume by using the headphone socket the speakers are automatically muted.

Is there anyway I can get round this and have both input and output when using an external mic?

Thanks in anticipation of help.