How to create a swept / chirp signal between a given starting audio signal and a given ending audio signal I can create a swept / chirp signal using matlab / octave with the code below (also see spectrum taken).  But how can I create a swept / chirp signal between a given starting audio signal and a given ending audio signal?  (One way to look at it is morphing one image into another image but using two audio signals)  or a sliding / chirp phase vocoder

clc,clear all
swept_startfreq = 500;
swept_endfreq = 225;
fs = 8000;
swept_dursec = 15;  % duration of signal in seconds  
swept_t = (0:swept_dursec*fs)/fs;  % Time vector

if (swept_startfreq == swept_endfreq)
      swept_phase = swept_startfreq * swept_t;
else
    swept_alpha = abs(log(swept_endfreq/swept_startfreq))/swept_dursec;
    swept_b = (swept_startfreq - swept_endfreq)/(1 - exp(-swept_alpha*swept_dursec));
    swept_a = swept_startfreq - swept_b;

    swept_phase = swept_a*swept_t - (swept_b/swept_alpha)*exp(-swept_alpha*swept_t);
endif

swept_sig_out = .8* cos(2*pi*swept_phase); %equation used to create signal
wavwrite([swept_sig_out'] ,fs,32,strcat('/tmp/swept_sig_out.wav'));  % export file


The frequency spectrum of the original code / chirp or swept signal it creates


So I would have an initial audio signal (use as the initial audio signal for testing) and an ending audio signal (use as the ending audio signal for testing).  How can I do a "morphing" between these two signals?

Here's what an example of the final file would sound like example of final audio signal when processed (please not this isn't exact but it shows how the pitch gradually changes from the original signal to the end audio signal over time  If you would like to know how I created the final file I used the initial signal and Audacity with the effect - Sliding Time Scale/Pitch Shift to get as close as possible to what the ending signal sounds like.

Added from comments: Start with the frequency with the largest amplitude in the initial signal and "morph" it towards the frequency with the largest amplitude found in the ending signal

It would sound like "one one one one one" gradually decreasing or increase in pitch depending on the ending audio signals largest amplitude frequency found.   Please note the Initial and ending audio signal may not be the same type of signal, the initial signal could be "one one one one" and the ending signal could be "two two two two"

Ps:  I'm using Octave 4.0 which is like matlab
As noted by endolith in the first comment, the problem is not well defined, in that, there's no one right way of doing this audio morph/crossfade. 

One method would be to use some form of interpolation between the source and destination spectra as discussed in this paper and implemented in this toolbox. A similar technique is also discussed in this paper.

This paper proposes another possible method inspired by the problem of heat diffusion. Imagine the left edge of a metal plate is held at a "temperature profile" that resembles the magnitude spectrum of the first signal and the right edge is held at a "temperature profile" resembling the magnitude spectrum of the second signal. The 2D "temperature profile" function between the two edges is reconstructed by solving a heat diffusion problem. An intuitively pleasing property of crossfades obtained with this method is that it produces smooth chirps if two sinusoidal frequencies are close to each other, but produces a fade-in/fade-out effect if they are farther apart. See their audio examples here.

I have not succeeded in locating good Matlab implementations of audio morphing algorithms. But there are a couple of good starting points. For instance, the Java library in the Spectral Toolbox has an audio morphing Java source code which can be converted to Matlab with some effort. Another good starting point might be this Github repo.

W. Sethares, J. Bucklew, Kernel techniques for generalized audio crossfades, Cogent Mathematics 2(1), 2015.

W. Sethares et al., Spectral Tools for Dynamic Tonality and Audio Morphing, Computer Music Journal 33(2), 2009.

M. Slaney, et al, Automatic Audio Morphing, Proc. IEEE ICASSP, 1996.