Go to Start > Help and Support. When the Help screen appears, type 'System restore' in the search box. Choose to restore system to an earlier date. From the calendar choose a date when you know (or hope!) the sound was ok. The computer automatically restarts with the settings in force at the earlier chosen date.
If this does not work, in the Help search box type 'Troubleshoot sound' and go through all the questions - hopefully this will help cure the problem.