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 (MuseScore→Preferences→I/O on the Mac). 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 could in that case also try a 32-bit MuseScore version.
In the "Preferences" dialog under the I/O tab is a button that says "Restart Audio and MIDI Devices" Clicking this button often fixes audio not playing.
On windows, right click the speaker icon, select "Open volume mixer" and ensure that your MuseScore version is listed there (you may need to scroll left or right) and that the volume is not muted.
If you have a newer Mac with a touch-bar, you probably already know all too well that it is very easy to accidentally hit the "mute" touch-panel-button while reaching for any key in the upper-right corner of the keyboard. Always try that (touching it to see if it shows the muted icon or not, twice if it does) first!
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.
Using bluetooth headphones: In some cases it might also work to toggle (press twice) the MIDI input button on the main toolbar or just restart MuseScore.
For a temporary fix, whenever this white noise occur, go to Start > Services > find "Windows Audio Endpoint Builder" > hit Restart.
For some Mac users a PRAM (Parameter Random Access Memory)/NVRAM (NV for non-volatile) reset may solve it