Below is a workaround to get the stft plot into your UIAxes. Please see app attached.
freq= 30;
[y,Fs] = audioread('handel_audio.wav');
y = fmmod(y,freq,1000,100);
plot(app.UIAxes,y);
stft(y,Fs,'Window',kaiser(1024,5),'OverlapLength',220,'FFTLength',4096);
h = gca;
app.UIAxes2.XLabel = h.XLabel;
app.UIAxes2.YLabel = h.YLabel;
app.UIAxes2.Title = h.Title;
app.UIAxes2.XLim = h.XLim;
app.UIAxes2.YLim = h.YLim;
for ii = length(h.Children):-1:1
h.Children(ii).Parent = app.UIAxes2;
end
cb = colorbar(app.UIAxes2);
ylabel(cb,"Magnitude (dB)")
colormap(app.UIAxes2,'jet')
view(app.UIAxes2,-45,65)
close(gcf)
