From what I just read, these SSS sounds are called Sibilance and are located in the 5 to 8 kHz range.
So I tried the Equalizer: Parametric filter in Shotcut and it worked pretty well with these settings (I only used Band 2). You may need to experiment with the frequency, gain and bandwidth on your own audio file
If you can treat the audio in an outside app, there is a group of plugins for Audacity called the Fish fillets that seams to work quite well also.
http://www.digitalfishphones.com/main.php?item=2&subItem=5
I did not test the plugins myself, so you’re on your own with that one.