If you see audio activity on the Master Bus channel and don't hear anything coming from your speakers, the issue is most likely in the audio driver or device setup or configuration or speaker setup. You'd probably want to check those things first, make sure nothing haschanged in your audio device driver setup and that the audio output is going where you think it's going.
Also, change audio devices to try to verify the above. If the built in audio device plays but not (for example) a USB device, or vice versa, there's a high chance that something is incorrectly configured or routed.