Simulate a simple multipath channel for OFDM Modulation test. MATLAB
Wav files expect values from -1 to +1.  You are probably exceeding those limits when you filter the signal through the channel model.  After the channel filter add the following:

after_channel = after_channel / max(abs(after_channel)); 

I suspect, though, that you will still have a problem.  I believe that wavwrite ignores the imaginary part of complex data, so you will be losing that part of the signal.