Please check to be sure you have the file ΓÇ£QuickTimeΓäó Musical InstrumentsΓÇ¥ in your ΓÇ£ExtensionsΓÇ¥ folder, because MarathonΓÇÖs background music will sound really, really stupid without it.
Be sure that your Cybermaxx helmet is properly hooked up to the serial port and turned on.
Be sure your external speakers or headphones are connected properly, and that you have enabled stereo output from the Sound Control Panel.
