Have you looked under Edit>Preferences>Audio Output Mapping / Audio Hardware.
Do other applications on the same system output audio?
What format is your source file that contains audio?
Is audio turned on in the Preview panel?
Please post a screen grab of your audio preferences in After Effects, and of your audio settings in Windows Control Panel or Mac System Prefs.
Are you still facing this problem? If not, let us know how you solved it. If so, please let us know so we can assist you further.