How to remove the hissing sound on Real Player

Open the Equaliser by pressing the button - or by opening the 'View' Menu and selecting 'Equaliser'

Adjust the 4k and 8k values to be zero (you can reset them by clicking 'Restore all default values') as shown below:

This should ensure that there is no hissing sound in the background. If this does not work, then adjust the other values until you reach a good sound. You can save your settings for future use.