This article covers not just earphones but also bone conductive headsets.
Contents
Germany
Illegal to wear earphones while bicycling. No laws specific against bone conductive headsets.
California
Illegal to wear earphones while bicycling. No laws specific against bone conductive headsets.
Delaware
Illegal to wear earphones while bicycling. No laws specific against bone conductive headsets.
Maryland
Illegal to wear earphones while bicycling. No laws specific against bone conductive headsets.
References
Bicycle hearing laws by country Wikipedia(Text) CC BY-SA