First and foremost, be sure you're using the Discord program/app or a working browser such as Chrome or Firefox, etc. (NOT the Steam browser)

A browser may be convenient, but may not have all of the features that the app itself has.

Mic/Voice Settings

Guide here

Mobile Problems

Sometimes when you connect to a voice channel, members may not hear you.

When that happens, disconnect and reconnect again. Sometimes it takes 3 tries.

If that doesn’t work, there may be an issue with your connection or the app.