So with the new Mixer API in SDL3, there seems to be no easy way to do things like ducking. Fading up or down from one volume level to another. the Props for MIX_PROP_PLAY_FADE_IN_MILLISECONDS_NUMBER, are useless because they enforce a fade from 0 to the specified track gain level.
but it would be very convenient to be able to do fades like this, with features such as MIX_PROP_PLAY_FADE_IN_MILLISECONDS_NUMBER and MIX_PROP_PLAY_START_FRAME_NUMBER already in place its not a huge step further to go from non-zero track gain to specified track grain when using MIX_PlayTrack