RCA 3 jack yo USBC Adapter for speaker system

I’m sure you can find USB sound cards which bring out two RCA jacks (stereo left and right), or a stereo mini-jack into which you can plug a adapter cable (readily available at places like Best Buy, Target, and Walmart).

The question is, why are there three RCA jacks?!? Is this speaker system meant to be fed with left and right stereo, plus a signal for a subwoofer?!?

There are also USB-C to TOS optical audio adapters. How well they work?

As for getting 5.1 audio — that usually happens via HDMI, and you don’t have that here (directly).

Maybe this, which lists macOS and 7.1: https://www.startech.com/en-us/display-video-adapters/cdp2hdmm5mh

There are several options for connecting this speaker system, depending on whether you are going for stereo or multi-channel sound (and depending upon what your source is capable of providing).

If you were just running stereo to it, I believe that you would only run two RCA cables, and that the system would handle the division of frequencies between the smaller speakers and the subwoofer.

The following links may be helpful:

Logitech – Z906 5.1 Surround Sound Speaker System

• THX® Certified Dolby Digital 5.1 decoding DTS decoding 3D Stereo
• Digital optical input: 2 (up to 1080p) 
• Digital coaxial input: 1 
• Six-channel direct input: 1 
• RCA input: 1 
• 3.5 mm input: 1 

Logitech Support - Connecting a computer to the Z906

Logitech Support – Getting Started – Speaker System Z906

If you look in the Z906 manual it shows the following inputs:

• 3x 3.5mm inputs - 6-channel analog PC direct (black, green, tan)
• 2x RCA inputs - 2-channel analog stereo (red, white)
• 2x Toslink inputs - 6-channel digital optical, each
• 1x Coaxial input - 6-channel digital optical

Your choices would be:

• Stereo audio cable from the iMac headphone out to the Z906's 2-channel input. You would need a cable like this one.
• External sound card device like the Creative Labs SoundBlaster X4 or similar connected to one of the Z906's optical inputs.

How do you intend to use this? Are you trying to build a home theater using your iMac as the media center, streaming video & audio from the iMac to a TV + Z906? If that's the case then there is an HDMI possibility that would involve using both the USB-C Digital AV Multiport Adapter plus an HDMI audio extractor like this one and of course appropriate cables.

This UGREEN USB-C tomRCA stereo adapter might work:

https://www.amazon.com/UGREEN-Adapter-Auxiliary-Speaker-Compatible/dp/B08NVPB15G