Check the following settings:
a) Audio sample rate in your After effects preferences.
b) Settings in your audio output device. AE does not play anything beyond 48 khz and it will only do so in stereo at the most. Turn off all surround and DSP settings, that your audio chipset may provide. If it uses a special driver, try to get rid of it or at least temporarily disable it and rely merely on Windows’ DirectSound and standard audio routines.
c) Limit the preview range by shortening the work area (the fat bar at the top of the timeline will do that for you). If it exceeds a certain length (depending on how much RAM you have) it will not play the audio (though AE builds the preview).
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