According to the digest of motorcycle laws by state in the BMW MOA "Anonymous" book, the following are the rules for helmet speakers by state.
Alabama - Legal
Alaska - Legal
Arizona - Legal
Arkansas - Legal
California - Legal, for one ear only
Colorado - Legal
Connecticut - Legal
Delaware - Legal
Florida - Helmet Speakers allowed, but not headphones
Georgia - Legal for communication purposes only
Hawaii - Legal
Idaho - Legal
Illinois - Legal for communication purposes only
Indiana - Legal
Iowa - Legal
Kansas - Legal
Kentucky - Legal
Louisiana - Legal
Maine - Legal
Maryland - Legal for one ear only
Massachusetts - ILLEGAL TO USE OR POSSESS
Michigan - Legal
Minnesota - Legal for one ear only
Mississippi - Legal
Missouri - Legal
Montana - Legal
Nebraska - Legal
Nevada - Legal
New Hampshire - Legal
New Jersey - Legal
New Mexico - Legal
New York - Legal for one ear only
North Carolina - Legal
North Dakota - Legal
Ohio - Legal
Oklahoma - Legal
Oregon - Some Cities have laws banning speakers of any kind
Pennsylvania - Legal for communication purposes only
Rhode Island - ILLEGAL TO USE OR POSSESS
South Carolina - Legal
South Dakota - Legal
Tennessee - Legal
Texas - Legal
Utah - Legal
Vermont - Legal
Virginia - ILLEGAL TO USE OR POSSESS
Washington - Legal
West Virginia - Legal
Wyoming - Legal
Washington DC - Legal