How can I finish this difficult assignment?

I was assigned this work that I found rather difficult. Now I get error messages after having seemed to solve half the assignment. The assignment is "sound waves under the water" on page 16

http://www.csc.kth.se/utbildning/kth/kurser/DN1232/berva13/Lab3BuppgJO.pdf#page=16

I have 3 matlab scripts to solve the problem, first I make a non.linear approximation that I think is correct.

z=[0:500:4000 5000:1000:12000]; % step is 500 in the beginning and then step is 1000 
data=[5050 4980 4930 4890 4870 4865 4860 4860 4865 4875 4885 4905 4920 4935 4950 4970 4990];
fun=@(p)(4800 + p(1))*ones(size(z)) +p(2)/1000*z+p(3)*exp(p(4)/1000*z)-data;
x0=[0 0 0 -1]; % Guess
opt = optimset('MaxFunEvals',1000);
p=lsqnonlin(fun,x0,[],[],opt);

fitf=@(t)(4800 + p(1))*ones(size(t)) + p(2)/1000*t+ p(3)*exp(p(4)/1000*t);
tt=linspace(0,12000,1000);
plot(z,data,'r-',tt,fitf(tt),'b-');
p;

Having run the above code the plot with the approximation looks ok. Then my problem arises when trying to solve the differential equation. I have this program

      function dZ=sys(x,Z)
      c=@(z)4800 + 5.66 + (14.58)*z/1000+ (257.4)*exp(-z/1000); % c(z)
      c=c(2000);

% Z(1):=z
% Z(2):=u
    dZ=zeros(2,1);    % a column vector
    dZ(1)=Z(2);
    dZ(2)=(c/cosd(7.8))^2*((1287*exp(Z(1)/1000))/5000 + 729/50000)/...
        (4800 + 5.66 + (14.58)*Z(1)/1000+ (257.4)*exp(-Z(1)/1000))^3;
    end

Then I can call the above program from another script

hold on
x=0:0.5:7000;

for i=-10:14 
%i=-10;
[X,Z]=ode45(@sys,x,[2000 tand(i)]);
plot(X,Z(:,1),'r',X,Z(:,2),'b')
i=i+1;
end

hold off

I get error message and I'm not sure if I understood it correctly how to solve the assignment. Could you tell me what should be done?

Warning: Failure at t=4.036432e+03.  Unable to meet integration tolerances
without reducing the step size below the smallest value allowed
(7.275958e-12) at time t. 
> In ode45 at 309

[Star Strider]

The warning most likely means that you have a discontinuity there. (I didn’t run your code.) Plot what you have to see what it is doing.