You may wear earphones in both ears:

To prevent hearing loss caused by exposure to wind and engine noise, you may choose to use ear protection while riding your motorcycle. You may not wear headset coverings, earplugs, or earphones in both ears unless they are specifically designed to reduce harmful noise levels. The headset coverings, earplugs, or earphones must not inhibit the wearer's ability to hear important sounds, such as horns and sirens. In-ear speakers (ear buds, etc.) are not legal in Washington.