Problem encoding and decoding a message in an audio file

Hello everyone. As the question says, I have encountered a problem while running the codes below. The goal of these codes is to input a message, encode it using the Vigenere method and then replacing the last bit of the sound wave's amplitude with the corresponding encoded message transformed to binary. So for example, we have the message 'Hello', we encode it using Vigenere with the key word 'maths' obtaining another string of characters, then we first transform each character to ascii and then to the corresponding binary, taking each of those bits and putting them in the audio file one by one, leaving a space of 16 bits between one letter and the following. I think that the problem has to do with the saving or the reading of the audio file, as if some of the bits introduced changed somehow, but I don't know why this happens.

Thank you very much.

Merry Christmas and Happy New Year!!

Here go both codes. There are plenty of lines uncommented, so feel free to ask if there's anything you don't understand.

Encoding

clear all
clc

[road,fs]=wavread('road.wav');
left=road(:,1);
right=road(:,2);
tamano=length(road);
mensaje=input('Introduzca el mensaje a codificar') % We ask the user to write the message to encode
ascii_mensaje=mensaje-0 % We convert the input to the equivalent ascii 

% Vigenere encryption

ascii_mensaje=ascii_mensaje-31; % We want to avoid getting a modulo between 0 and 31, as these do not belong to the printable characters.

clave='mates'-0-31;%palabra clave en ascii
for i=0:length(clave):length(ascii_mensaje)
  if (length(ascii_mensaje)-i)<length(clave)
  codigo_vigenere(i+1:length(ascii_mensaje))=mod(ascii_mensaje(i+1:length(ascii_mensaje))+clave(1:(length(ascii_mensaje)-i)),127-31);
  else   
  codigo_vigenere(i+1:i+length(clave))=mod(ascii_mensaje(i+1:i+length(clave))+clave,127-31);
  end
end

codigo_vigenere=codigo_vigenere+31

ascii_mensaje_binario=dec2bin(codigo_vigenere)
size1=size(ascii_mensaje_binario);

fila=1;
time=(1/fs)*length(left);
t=linspace(0,time,length(left));
plot(t,road)
negativo=1;

for i=1:16:tamano

  for j=1:7
      
        if road(i+j-1,1)<0
           negativo=-1;
        end
           
          aux=dec2bin(abs(round(road(i+j-1,1)*10000)))   
          aux(end)=ascii_mensaje_binario(fila,j)
          
          road(i+j-1,1)=negativo*bin2dec(char(aux))/10000;

            negativo=1;
        
        end

        fila=fila+1;

        if fila>size1(1) % We break the loop once the message has been placed in the audio file.
         break
        end
  end      

wavwrite(road,fs,'prueba(modificada).wav');

Decoding

clear all
[road,fs]=wavread('prueba(modificada).wav');
left=road(:,1);
right=road(:,2);
tamano=length(road);
fila=1;

time=(1/fs)*length(left);
t=linspace(0,time,length(left));
plot(t,road)

for i=1:16:tamano 

   for j=1:7
          
            aux1=num2str(dec2bin(abs(road(i+j-1,1)*10000)));   
            ascii_mensaje_binario(fila,j)=aux1(end)
            
    end

        fila=fila+1;

        if fila>size1(1)
     break
   end
  end  

ascii_mensaje=bin2dec(num2str(ascii_mensaje_binario))'

comp= char(ascii_mensaje)

% Vigenere Decoding
ascii_mensaje=ascii_mensaje-31; 
clave='mates'-0-31;

for i=0:length(clave):length(ascii_mensaje)
  if (length(ascii_mensaje)-i)<length(clave)
  codigo_vigenere(i+1:length(ascii_mensaje))=mod(ascii_mensaje(i+1:length(ascii_mensaje))-clave(1:(length(ascii_mensaje)-i)),127-31);
  else   
  codigo_vigenere(i+1:i+length(clave))=mod(ascii_mensaje(i+1:i+length(clave))-clave,127-31);
  end
end

codigo_vigenere=codigo_vigenere+31;

char(codigo_vigenere)

[Miguel Ángel]

Any ideas?