Download and install Audacity. This is a free audio editor that provides professional-level results.
Open Audacity and drag the music file that you wish to edit into the main preview pane. This will enable the program to analyze and import the audio for editing.
Select a portion of the audio in which there is only hissing. This should be at the beginning or end of the audio.
Select "Noise Removal" from the "Effects" menu. Click "Get Noise Profile." The audio editor will then assess the selected portion of audio from Step 3 to differentiate between hissing and audio.
Select "Noise Removal" from the "Effects" menu again. This time, just click "OK." This will run the noise-removal function on the audio.
Select the "Low Pass Filter" option located under the "Effects" menu. Low-pass filters remove audio that is over the specified frequency. Leave the option as is and press "OK."
Select the "Compressor" under the "Effects" menu. Don't change any of the options and press "OK." The compressor will help to further minimize any of the remaining hissing.