Test of an in-ear microfoon in my helmet
 

In this video I test 'Roland WPM-10 In-Ear Microphones for GoPro 3' in my helmet during a ride on my Honda GoldWing GL1200. (English subtitles)

So that's one I don't need to bother with then. A pity as I have a 'spare' GoPro Hero 4 that I think the test microphone works with.

Sound is always a problem with on bike videos and I seem to remember that you previously used a separate sound recorder / microphone (can't remember which one) and then (presumably) added the sound track to the video afterwards. Is that still your preferred way of doing it?

I use a small microphone that I fit in my helmet that I connect to the GoPro 3 on the handlebars. That works reasonable good.
I hoped the in ear microphones would be better but they are not.

Inear mics will never work properly with a clear sound due to the fact that their mic is covered by the helmet material!

In general best type of mic should be a Lavalier mic.
E.g. one like this:
https://www.syncoaudio.com/products/sy-s6e-lav

You can connect by cable to the GoPro or you can go for a wireless setup.

For wireless you need a receiver and a transmitter to relay the sound from a mic to the camera.

E.g.: https://www.amazon.de/s?k=Rode+Wirel...vcamwiltr07-21

I use this one https://www.amazon.co.uk/R%C3%98DE-L...54&sr=8-5&th=1

With GoPro 9 with MediaMod.