I’m kind of new to this myself, so I’m not sure I’m looking in the correct place, but here’s some brief notes on my research so far:
Android has a MODIFY_AUDIO_SETTINGS permission that:
Allows an application to modify global audio settings.
On iOS there is a concept of “Audio Units.” There are several included audio units, here’s the few that may be helpful:
iPod Equalizer Unit:
Provides the features of the iPod equalizer.
3D Mixer Unit:
Supports mixing multiple audio streams, output panning, sample rate conversion, and more.
Multichannel Mixer Unit:
Supports mixing multiple audio streams to a single stream.
There’s a Swift sample project in the iOS Developer resources interfacing with the iPod EQ unit here: https://developer.apple.com/library/content/samplecode/iPhoneMixerEQGraphTest/Introduction/Intro.html#//apple_ref/doc/uid/DTS40009555
This person also seems to have used standard React Native to interface with audio and use equalization: https://www.youtube.com/watch?v=2n4feu3O_zU
That’s what I’ve found so far, but not really sure I’m headed in the correct direction.