How to get sound working if you don't hear anything

Occasionally, when something about MuseScore's installation is changed, playback may stop working. This How-To describes how to get playback working again.

Check whether the cabling (between computer and speakers/headset) is still correct (unless it is bluetooth of course), no cable loose or broken (and yes, this can happen and has happened).
First (well second), 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!
If after trying all of the above steps (plus what is suggested below), then please ask for help on the Support forum. Be sure to say which version of MuseScore (Help→About) and what operating system you are on, what audio devices you are using, and what settings you see in Edit→Preferences→I/O after doing the revert to factory settings as per suggestion 2 above.

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. See this howto on sweetwater.

Using bluetooth headphones/speakers: In some cases it might also work to toggle (i.e. press twice) the MIDI input button (looking like a 5-pin DIN connector) on the main toolbar or just restart MuseScore, see also How to solve bluetooth audio problems

For a temporary fix, whenever this white noise occur, go to Start→Services, find Windows Audio Endpoint Builder→ hit Restart (Windows).

Actualisation (Start→devmgmt.msc→Update audio driver) may solve it (on Windows)
A PRAM (Parameter Random Access Memory)/NVRAM (NV for Non-Volatile) reset and/or audio driver may solve it (on Intel-based Mac)
Restart the Mac in safe mode by holding Shift at start-up time, and then reboot again.

There has been a recent report that, on Mac, Native Instruments' software Komplete Kontrol, resp. 2 of the agents it installs, NIHostIntegrationAgent and NIHardwareAgent, to cause this issue, cure was to disable them from running and restart.