Volume Setup Help

Hi everyone,

Alright so I'm hooking a PC 5.1 system (a decade old Altec-Lansing ADA995) into my 2012 Sony tv. Based on the last few days of research I've concluded my best way to make this work is as follows:

TV Optical Out -->
Digital-Analog Converter (Optical cable in, 6x RCA out) -->
Red/White RCA - 3.5 converters x3 -->
Yellow/Green/Black 3.5s that plug into the speaker system

If thats confusing, I'll put it into words: The Altec's use the triple 3.5 cords (Green/Yellow/Black), meaning I have to convert them into RCA format (hence the three converter cables - each 3.5 gets split into a double RCA, leaving me with the 6 RCAs) which are then plugged into a Digital-Analog Converter (DAC), which an optical audio cable is feeding from the DAC back into the TV (or xbox, etc). However, this setup leaves me with no volume control, since the Altec's have no external volume control (by original design)

My question is this: What can I insert into this setup to get volume control, and where would it go in the chain?

Extra detail: I have used these as speakers hooked up through Apple airplay (a 3x 3.5 splitter that the Yellow/Green/Black were plugged into that I then plugged into an Apple airport) and I could control volume through iTunes with that setup, so I do know that they respond to volume control.

Thanks in advance! Sorry it's a tad confusing