Controlling External Speaker Volume on Sony Bravia BX420 with Remote?

I just purchased a 32" Sony Bravia BX420 and hooked up my Dell PC speakers using a aux-to-RCA (red/white) converter cable. I hooked up the red and white RCA cables to the audio out on the back of the tv and have the sound working fine after I've set the settings to "speakers." Problem is, when I change the volume up/down/mute using the remote, no change is made. The only way to control the volume is to physically turn the knob on the PC speakers' main speaker. Is there any way i can control the volume using my Sony remote? I don't want the remote to directly work with the speakers; just for them to be able to control the TVs volume output to the speakers. My older Toshiba did this no problem (same speakers). I'd turn the speakers on and use my TV remote to control the output of the speakers (the max volume would be the amount I had turned the knob on the speakers. If the knob wasn't turned, there would be no sound). The TVs audio out is Fixed so I don't have the option for changing it to variable. Is there any other way to control the volume on an audio fixed TV? Thanks in advance. I really appreciate all the help. :)