Couple of things you could try:
1 - If the M is red like in the picture below, it means the audio bus it's muted.
2 - If you have multiple audio buses, and the S is pressed in any of them, it's in solo mode, and will mute all the other buses.
3 - If you have some effects or chains, the B pressed will bypass any links between buses you previously had.
4 - If you change the settings, or effects, or edit the buses in any way through scripts, post the codes for us to take a look.
5 - See if your audio nodes (audiostreams) are directed to the corrected buses, there's a bus property in every audiostream, which by default is set to master.
6 - Check if you have sound in your system, hardware-wise.
7 - Check your options in the audio tab for your project.
If anything above doesn't work, here's the godot doc for the audio buses: https://docs.godotengine.org/en/3.2/tutorials/audio/audio_buses.html