All fine, we’ve been all new here
I don’t know why I got the idea that you were talking about colours, it was late so I guess I confused myself. The thing is it depends on what you are working with, but if it’s a plugin you probably have an AudioProcessor and in the processBlock function (where you process all the audio/dsp code) you can do it. It should be something like this (brief idea without testing):
for (all the samples to process)
outputBuffer [i] = bufferA[i] * fadeCoefficient + bufferB[i] * (1.0f-fadeCoefficient)
Where fadeCoefficient must be a float from 0.0f to 1.0f otherwise you will start clipping since your outputBuffer values will be higher than 1.0f (or lower than -1.0f). And here you must take into account the sample counting: i.e if you do the transition in 4 processBlock calls, your fadeCoefficient variation would be 1/4 = 0.25f. That would be in your 1st iteration fadeCoefficient = 1.0f, then 0.75f, then 0.5f, then 0.25f or just 0.75f, 0.5f, 0.25f, 0.0f.
Then you add your outputBuffer to the audioBuffer of the processBlock.