After much frustration unsuccessfully trying to find a USB-C solution for my high-end earphones with detachable MMCX connectors, I decided to piece together a cable using a re-purposed USB-C digital-to-analog converter and a 3.5 mm to MMCX cable.
Step 1: Enter the AIAIAI C60
AIAIAI is a Danish company that makes a series of modular headphones. One of their modular parts is a USB-C to 3.5 mm (ironic, I know) adapter for their headphones for $40 + $10 shipping. I was optimistically curious about what I might find inside this adapter, so I bought one.
Step 2: Opening Up the C60
After using a fingernail to split the case apart, I found what I was hoping for, easily-accessible solder pads.
Step 3: The Treasure Inside
After removing the black glue and de-soldering the wires for the 3.5 mm plug, I found four, nicely-labeled, solder pads for L+, L-, R-, and R+. I then cut my MMCX cables to an appropriate length and soldered the left and right channels to the appropriate pads taking care to keep the wires out of the way so that the case could re-close.
Step 4: In Business
After testing that everything was working, I closed up the case and used a tiny dab of superglue in each corner and in the middle of each side. It sounds, looks, and works great!