Frequency Modulation to arbitrary Audio signals I understand how FM synthesis works, but I was wondering how to FM arbitrary samples or part of samples. I.E any frame of audio samples.

Further more the modulator can itself be an arbitrary frame of samples itself.

How is this done in an algorithmic manner?
I have, in the past, used delay modulation on arbitrary audio signals.  but the modulation waveform itself was not arbitrary, but was a sinusoid.  but we could speed this modulation oscillator up to an audio rate.

Now that I think of it, I tried hooking that modulation oscillator up to a pitch detector.  This is a quarter century ago, so I don't remember everything.

I s'pose you could derive that delay modulation signal from the input and likely LPF it before applying it to the instantaneous delay of the signal going out.