First, check the microphone outside OBS
Open the microphone test and speak. If its meter moves, the microphone and your computer work, and the fault is inside OBS: carry on below. If it stays silent there too, fix the system first; the test lists the Windows and Mac steps.
On Windows, OBS counts as a desktop app, so Let desktop apps access your microphone must be on under Settings, Privacy & security, Microphone. On a Mac, open System Settings, Privacy & Security, Microphone, and turn OBS on in the list.
Make sure OBS is listening to the right device
OBS takes microphone sound in one of two places: Settings, Audio, where Mic/Auxiliary Audio sets a microphone for every scene, or an Audio Input Capture source inside a scene. Both have a Device list, and both start on Default.
Default follows whatever your system is set to use. If that is a webcam’s built-in microphone or a headset you are not wearing, OBS listens to that instead. Choose your microphone by its name, speak, and watch its meter in the Audio Mixer.
Check the source is in the scene you are showing
An Audio Input Capture source only captures while it is active, which means while its scene is on. A microphone added to your starting scene goes quiet when you switch to a gameplay scene that does not contain it. Add it to every scene you use, or set it once under Settings, Audio instead.
Check the OBS mixer
In the Audio Mixer, a crossed-out speaker icon means the source is muted, and a fader at the bottom means it is silenced. Speak and watch the meter. If the meter moves but your recording is silent, the microphone is not on the track your recording or stream uses; the stream and recording tracks guide shows where that is set.
An XLR microphone through an interface
Choose the audio interface as the device, not the microphone: your computer only sees the interface. Turn up the gain on the interface and, for a condenser microphone, switch on 48 V phantom power there.
If the meter in OBS moves only on the left, the microphone is in input 1 of a two-channel interface, and viewers would hear you in one ear. Turn on Downmix to Mono for that source in Advanced Audio Properties. The microphone test shows a one-sided input before you open OBS.
If you hear the microphone twice
The same microphone set in Settings, Audio and also added as a source in a scene is captured twice, which sounds like an echo. OBS’s own advice is to disable the device globally before adding it to scenes. An echo in your headphones only, not on the recording, is a monitoring problem instead: the headphone test checks for it.
Prove it with a recording
Record ten seconds in OBS with your normal scene, speaking at your usual distance, then play the file back. OBS’s own guide ends its level advice the same way: record a session as you normally would and listen before going live. Then set the level with the microphone test’s meter, which uses the same zones as OBS.