Answer
On Apex Legends PC, open Settings, go to Audio, and check the Voice Chat options.
The EA accessibility features article treats Voice Chat as part of PC settings. Adjust input / output device, volume, and push-to-talk vs open-mic behavior for your device and OS environment.
Caveats
Voice chat behavior also depends on OS permissions, headset, drivers, and whether another app such as Discord is holding the device. In-game settings alone cannot diagnose every issue.