During the startup process, sometimes there is not enough system memory available to play both the video and sound. When memory is low, Quicktime drops the audio portion of the movie.
QuickTime version 2.0 fixes this problem. If you are experiencing this problem you should upgrade to version 2.0 to correct it.
If you have any version of QuickTime prior to 2.0 and you cannot get version 2.0 right away, an intermediate workaround would be to place a startup screen file in the system folder in addition to the QuickTime movie in the Startup Items folder. This will cause the system to allocate more memory at startup time and allow QuickTime to play both the video and audio portions of the movie.