Why am I not able to hear the audio when playing a video file? 

This is a Windows setting that is muting RealPlayer from playing audio.

To resolve your issue, please follow these steps.

1.  Play any audio or video file with RealPlayer

2. Right click on the volume icon in your task bar as shown in the screenshot below:

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​3. You will receive a few options while the very first one will be Open Volume Mixer

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4. You will see RealPlayer here as shown in the screenshot below

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If the volume ticker is all the way down, you will need to move it up.