Usually, headphones and headsets come with a fixed cord, with an application-specific connector at the end. This limits their use for one given set of applications - a cellphone handsfree, a laptop headset..., places further limits on the length of the cable (sometimes it is too short, sometimes the same length is annoyingly long)...
Not least, the cable is the most damage-prone piece of the equipment. Splitting it off the headset itself means a cable damage can be handled with a quick replacement for a spare, without having to carry an entire spare headset.
As an universal connector for the headphones side, a USB-B connector was chosen on the basis of robustness, reliability, availability and cost. Four terminals are enough, if the ground line from the microphone and from the headphones is shared. Which is fortunately the usual case.
The pinout was chosen as follows:
USB B connector
(from bottom)
----------------. USB headset
`----------------'
A simple but decent headset, the Canyon CNR-HS1, was chosen as one of the candidates for the surgery. The cord was cut off, a hole was filed in the casing, a USB connector was attached to the wires and hot-glued in place.
 Canyon CNR-HS1 headset |  Disassembled headphone |  Hole for USB connector |  USB connector, seated in |
 USB connector, seated in |  Wiring |  Wiring, connector attached |  Connector connection detail |
 Connector glued in situ |  Connector in situ, other side |  Finished headset |  Finished headset, cord attached |
 Finished headset, cord attached, detail |
Higher-quality headset, Koss SB/45, was also modified. In contrast with the Canyon one, the sound quality is better, the volume is higher, and the size is way bigger. The choice of the headphones is therefore dictated primarily by the quality/logistics tradeoff.
 Koss headphones, inside |  Cord pass |  USB connector mountpoint |  Cord detail |
 Connection to the connector |  Connector in situ |
As specified above, the headsets allow use of various custom-made cords, differing in length and the other-side connectors. One cable was made from a deceased Nokia handsfree, as the sourcing of the 4-pin 2.5mm jack from other source was difficult. Another was made from the original cable from the Canyon headset, with separate headphones and microphone jacks and a volume control in the cord.
 USB connector, cable with strain reliefs |  Black cord, USB connector |  USB connector, strain relief in situ |  Laptop-headset cable |
As the relatively long handsfree cord was somewhat annoying when the headphones were moved around and stored and otherwise not in direct use, it was decided to build a retractable cord.
The retractable cord was sourced from a deceased laptop mouse.
A simple LC low-pass pi-filter with cutoff frequency at several 10s kHz was placed in the phone-side connector body in the microphone line, to alleviate the issue with buzz coupling into the cord when talking on the phone. It proved to be fairly functional, though not ideal.
As the cord retraction mechanism was fairly big and bulky, in comparison with the thickness of the cord, it had to be fixed in position when in use to eliminate its tendency to flap around. A cable clip was hot-glued to its body, so the mechanism can be clipped on a shirt or to the strap the phone is carried on.
 Mouse, the cord donor |  Mouse, the cord donor, opened |  Connectors |  Phone-side connector assembly |
 Phone side connector assembly |  Phone side connector assembly |  Phone side connector assembly |  Headphone side connector |
 Headphone side connector |  Headphone side connector, assembled |  Headphone side connector, assembled |  Phone side connector, assembled |
 Retractable handsfree cord |  Retractable handsfree cord |  Strap clip |  Strap clip, detail |