probable PCM problem with FFT noise added after wavwrite

Hi all,
I am fairly new to MatLab. I am trying to do a project to separate audio.
I have generated to tones, C0 and G0, and added them, checked the fft (two clean mirrored spikes, as expected) and written them to a wav file.

when I read the file back and check the FFT it shows a load of noise between the two spikes and between their mirrors, took this to be some kind of aliasing so increased the bits per sample, which seemed to work, but also seems to significantly reduce the amplitude and phase.

I haven't been able to get much further with this so any help is much appreciated.

thanks

Here are the two programs I am using.

Create_tones.m

t = [0:0.0001:1]; % Set independent (time) variable.1
A = 1; % Set amplitude.

%f_c = 261.63; % Create C tone.
f_c = 16.35;
tone_c = A*sin(2*pi*f_c*t);

%f_g = 392.00; % Create G tone.
f_g = 24.50;
tone_g = A*sin(2*pi*f_g*t);
subplot(2,1,1);plot(tone_c);
subplot(2,1,2);plot(tone_g);
Fsamp = 44100; % Set sampling frequency.
%Fsamp = 8000;
bps = 32; % Set bits per second.
siz = 65536
wavwrite(tone_g, 'tone_g.wav'); % Write audio to file.
wavwrite(tone_c,'tone_c.wav');

tone_mix = (tone_c + tone_g); % Add tones.
wavwrite(tone_mix,Fsamp,bps,'tone_mix'); % Write mixed audio to file.
%sound(tone_mix,44100,bps); % Play mixed audio

fft_tone_c = fft(tone_c,65536); % Get fft of each tone and mixed tone.
fft_tone_g = fft(tone_g,65536);
fft_tone_mix = fft(tone_mix,65536);

n = [0:65535]; % Setup array for graphing against.
fp = n*(Fsamp/65535);
subplot(3,1,1); plot(fp,fft_tone_mix); % Plot each tone and mixed tone.
subplot(3,1,2); plot(fp,fft_tone_g);
subplot(3,1,3); plot(fp,fft_tone_c);

split_wav.m

file1 = 'tone_mix.wav';
file2 = 'split.wav';

siz = wavread(file1,'size'); % Get total length of mixed audio.
siz(:,2) = []; % Make siz vector an "int".

[x,Fs,bits] = wavread(file1, siz); % Read in mixed audio.

y = fft(x,siz); % Get fft of mixed audio.

z = ifft(y);
wavwrite(z,Fs,bits,file2); % Write mixed audio to file.
[a,Fs,bits] = wavread(file2); % Read mixed audio file.

n = [0:siz-1]; % Setup array for graphing against.
f = n*(Fs/siz-1);
plot(f,y); % Plot mixed fft.
title('Mixed FFT'); Start a New Thread