DIAGNOSIS The microphone isn't busted—this is almost always a permissions issue with the Tapo app on your phone, or the speaker on the other end is muted in the app.
FIX First, when you're in live view, tap the microphone icon. It should have a slash through it if you're muted. Tap it to unmute—you should see the slash disappear and you might hear a faint click from the camera. Now try speaking. If that icon was already unmuted, go to your phone's main Settings > Apps > Tapo > Permissions. Ensure the Microphone permission is set to "Allowed" or "On."
IF THAT DIDN'T WORK Do a quick test with a different phone using the same Tapo account. If two-way audio works on the second phone, the problem is with the first phone's app or settings. Try force-closing the Tapo app and reopening it, or uninstalling and reinstalling the app.
HEADS UP There's a slight delay in two-way audio, about a second or two. Don't expect a natural back-and-forth conversation. Speak clearly, wait, then listen for a response. Also, the microphone isn't super sensitive; you need to be within normal talking distance of your phone.