Apply Audio Compressor, Limiter, Equalizer, Surround Sound Virtualizer to Audio. The new audio dynamics API is used, and is available on Marshmallow and later. AFAIK this is the only app that uses this API as a system wide effects manager (!)

Following effects are available:
1. Compressor
2. Limiter
3. Equalizer

Presets are available, and you can use custom settings to suite your taste.

As a bonus, you can also launch the in-built android system equalizer using this app.