Hi,
The function you should use in the core API is
https://fmod.com/resources/documentation-api?version=2.1&page=core-api-system.html#system_createdsp
Then you would add it to somewhere in the mix graph. If you want to add it to a ‘channel’ (to modify existing data) you can use Channel::addDSP to insert it into a channel stream (ie a channel handle returned from System::playSound)
Another way to actually play it on its own channel, is to use System::playDSP if it is a generator/instrument type of DSP.
The 3rd way is to avoid channels, and just play it on a channelgroup so it is always running in the background regardless of what channels are doing. This can be done with ChannelGroup::addDSP
For an example of a DSP implementation, look in the example folder for dsp_custom where it simply modifies a signal by multiplying it by a gain value.