You can buy a USB dongle that gives you speaker+microphone (either two separate standard 3.5mm connectors or combined into a TRRS connector, whichever you want). If you will only want to use the microphone then the two separate connectors may be more convenient. Alternatively, you can pay more money and get a decent USB microphone.
Many USB webcams would have built-in audio capability, so you may not even need a separate external mic, provided that the audio quality is adequate.