iMac Sonoma 14.2.1. How to use external speakers with an AUX cable, NEW TECHNOLOGY??

I had a big iMac in 2015. Two years later, I added an Altec Lansing 4 speaker and subwoofer set, and sound was beautiful. The speakers plugged into the back of my iMac with an AUX cable.

I purchased a new iMac in June 2023. Surprise there is NO way to plug in my AUX cable in the back of my new iMac. So, I purchased from Apple a USB-C to headphone jack and connected my AUX cables to this USB-C to headphone jack. The volume is GREATLY DIMINISHED. I have checked all the connections. They are good.

What's wrong? Why is the volume so low? I have also checked all the sound volume settings and they are as high as they can go. Volume is loud if it is channeled through the iMac speakers. Is it just new technology and my speakers have gone the way of the dodo?

Any help would be appreciated.

Thanks!!