Starting in April, Japan gets stricter regarding violations while cycling.
Using earplugs is forbidden, and I'm out of ideas for alternatives.
I'm using a single earplug to listen to podcasts, with the FiiO M3K player running #rockbox - great device and excellent rockbox support.
The canonical idea to become "compliant" is to go for bone conduction headsets - just these are all Bluetooth connected, and Rockbox does not support Bluetooth. Hm..