How to use findpeaks in the frequency domain?

[audio_file, Fs] = audioread("honours_gun.wav");
L = size(audio_file,1);
Ts = 1/Fs;                                                      % Sampling Interval
Fn = Fs/2;                                                      % Nyquist Frequency
FT_af = fft(audio_file)/L;                                      % Fourier Transform
Fv = linspace(0, 1, fix(L/2)+1)*Fn;                             % Frequency Vector
Iv = 1:numel(Fv);                                               % Index Vector

[PksL,LocsL] = findpeaks(abs(FT_af(Iv,1))*2, 'MinPeakHeight',4E-3);
[PksR,LocsR] = findpeaks(abs(FT_af(Iv,2))*2, 'MinPeakHeight',4E-3);

figure
subplot(2,1,1)
plot(Fv, abs(FT_af(Iv,1))*2)
hold on
plot(Fv(LocsL), PksL, '^r', 'MarkerFaceColor','r')
hold off
ylabel('Amplitude')
title('Left Channel')
subplot(2,1,2)
plot(Fv, abs(FT_af(Iv,2))*2)
hold on
plot(Fv(LocsR), PksR, '^r', 'MarkerFaceColor','r')
hold off
xlabel('Frequency (Hz)')
ylabel('Amplitude')
title('Left Channel')