I've been riding with Sena SMH10 for over a year and like it a lot. I am not a fan of helmet speakers, and have been using their optional earbuds clamp almost exclusively. I decided to convert the stock helmet speaker clamp to earbuds to use with my other helmet. This clamp has a 3.5mm aux jack for use with audio sources without Bluetooth. My sole source is an Android smartphone, so I've never used the aux jack and it would serve me better re-purposed as an earbuds jack.
This is a factory earbuds clamp disassembled. The audio is fed to the earbuds though a pair of 75 Ohm resistors because earbuds need far less power than the helmet speakers.
I had 51 Ohm resistors handy, they also work. Audio is a bit louder, but not by much.
This how I did it:
To disassemble the clamp, remove two visible screws and two screws under the rubber cushion.
Then slide the microphone boom shroud towards the mike and the clamp will come apart.
I don't need the speakers, so I unsoldered and removed them, however it should be possible to have both speakers and earbuds working at the same time.
Remove the PC board, turn it over and cut four traces with a sharp blade or knife.
Sorry for the bad focus, I think the traces are still visible.
Solder resistors between the jack and "R+" and "L+" points. Probably surface mounted chip resistor will be a bit easier to fit in.
Solder the common wire between the jack and "L-"
Reassemble in the reverse order, correctly positioning the mike boom. (The triangle mark on the mike should point toward your mouth). I removed the speakers and used a "liquid gasket" silicone compound to waterproof the opening for the cables. If you don't want to do it, you can leave the speakers in place or cut their wires flush.