The Doppler effect is a real-life sound phenomena that shows up as a frequency change when either you the listener or the source of the audio is moving, its better explained here: Doppler Wiki.
This is a small mod that adds just that, with built-in compatibility for Simple Voice Chat and Plasmo Voice.
doppler_demo_crf23.mp4
YouTube upload: https://www.youtube.com/watch?v=dTkfkTWfMPQ
Default effect "scale" is at 40%, noticeable but not too exaggerated, though I havent tested all means of movement next to audio sources, so you're free to change it in ModMenu's settings screen for the mod or in the config file located at <minecraft>/config/doppler.cfg, value is a decimal with ranges from 0-1 (default: 0.4).
I kept seeing Instagram reels that have "doppler effect" caption with a clip of something in Minecraft just decreasing in volume, which is NOT the effect in question!! So I made this mod, so you can hear what it really sounds like, who will use it? idk, but its fun 😸