how to get back audio after filtering and applying ifft

clc;
clear all;

srate = 10000;
time= 0:1/srate:2;
f=500;
x1=sin(2*pi*f*time);    %random noise
%sound(x1);
subplot(421);
plot(x1);    %ploting noise  
title('signal1');

f=900;
x2=sin(2*pi*f*time);    %random noise
%sound(x2);
subplot(422);
plot(x2);    %ploting noise  
title('signal2');

signal3=signal1+signal2;
subplot(423);
plot(signal3);

swavex=fft(signal3)/npnts;
hz=linspace(0,srate/2,floor(npnts/2)+1);
subplot(424);
m1=2*abs(swavex(1:length(hz)));
%sound(m1);
plot(hz,2*abs(swavex(1:length(hz))));
set(gca,'xlim',[0 max(f)*1.3]);
xlabel('freq (hz)'),ylabel('amplitude (a.u)')
[m2,l]=findpeaks(m1)
s=size(swavex);
z1=[ones(1,600) zeros(1,19401)];
a1=swavex.*z1;
subplot(425);
stem(a1);
xlabel('freq (hz)'),ylabel('amplitude (a.u)')
title('filtered signal');

srate2=0.5;
time2=0:1/srate2:2;
npnts2=length(time2);
reconSig = ifft(a1);
hz=linspace(0,srate/2,floor(npnts2/2)+1);
m2=2*abs(reconSig(1:length(hz)));
subplot(426);
plot(hz,2*abs(reconSig(1:length(hz))));
set(gca,'xlim',[0 max(f)*1.3]);
xlabel('freq (hz)'),ylabel('amplitude (a.u)')
sound(m2);