How to restore sound if playback stops working
First, to verify the problem, go to the View menu and choose Synthesizer. By default (and as of MuseScore 2.2), you should see "MuseScore_General.sf3" in the Fluid tab of the Synthesizer window.
If it's not there, or if View→Synthesizer is actually disabled, one option is to restore MuseScore's factory settings. This will reset everything, not only the missing SoundFont.
Alternatively, esp. if View→Synthesizer shows you there is no SoundFont loaded, you can avoid reverting to factory settings by simply downloading and installing a replacement SoundFont. Make sure to apply the "Set as Default" button in the Synthesizer after loading the replacement SoundFont.
Check Edit→Preferences→I/O. If that is empty, try to update your sound drivers. Esp. if you're on a 64-bit Windows and MuseScore 2 has sound, but MuseScore 3 does not, the cause might be 32-bit sound drivers. You coud in that case also try a 32-bit MuseScore version.
Note: In case of interference with other programs, esp. on Windows, go into your soundcard configuration from Control Panel and ensure that the "Allow applications to take exclusive control of this device" in the Advanced tab of both Playback and Recording is unticked.
For some Mac users a PRAM (Parameter Random Access Memory)/NVRAM (NV for non-volatile) reset may solve it