What type of mac are you using?
Are you using the built in speakers or are you using external seakers?
Let us know
What type of speakers are your Tower speakers?
So you connected the monitors speakers using the included cable from the monitor to the G5 using a 3.5mm headphone jack, Is that correct?
If so could you open System Preferences and select Sound then click Output and select your preferred output method of Analog.
Let me know what happens.
Could you reboot your machine with the monitor speakers connected then once it has reobooted try going to the movie trailer site again and let me know what happens