Something that lets me select a frequency above which all sounds would be filtered in real time? I don’t know if a software can separate the high pitched sounds from the rest of the sounds. If it was possible that would be perfect. If not just removing all sounds when any sound goes above the threshold.
You could try EasyEffects (FKA PulseEffects)
The Equalizer plugin should work, but there may be a better one.
This is thé answer for real time in linux