The problem where the music plays too slow or fast in a video recording is caused by a WAV file that does not have the correct sample rate expected by showsim or by a custom compression setting that some programs use when creating the WAV file.  In the software that was used to assemble the WAV file it is important not to have any compression settings turned on.  The WAV file should be saved as a 44100 Hz (44.1 KHz), 16 bit stereo file with no compression.