System-wide sound effects on OS X?
You can add Audio Unit effects, which can be used by any AU-capable audio app. Example with Apple originals &amp; third party additions…

Generally, you cannot plug these directly into the Mac's audio output, you need an app capable of accessing them. Apps such as SoundSource can do this for any app or all, at simple general consumer level, intercepting between app or system output &amp; the intended hardware out.
There is, of course, already a large market for these effects as stand-alone processors, in the pro audio world. From free to expensive.
If you search &quot;VST plugins&quot; you'll find many of these that come in multi-format, VST, AU, etc. VST is a proprietary format from Steinberg, but the plugin API is available for developers. Once the effect is coded (waaay beyond my abilities) then they are often cross-ported to other formats.