ArcWear.com has performed testing on ear plugs in arc flash. This testing is included in e-Hazard.com electrical safety training classes such as the Low Voltage Qualified (NFPA 70E) class and the NESC, OSHA 1910.269 Line Mechanics and Meter Technician Class.
The results are that foam does melt but some foams may not ignite easily. Red foam tested ignited at less than 7 cal/cm² which the yellow tested didn’t ignite but did melt. Arc Ear Plugs did not ignite until >10 cal/cm². The only plug which did not ever ignite or melt was a custom ear plug made of silicone rubber. The NFPA 70E standard requires ear canal inserts but we have also tested some muffs and no muff is currently rated for arc flash exposure. If covered under a hood none of these forms of hearing protection should be an additional hazard so following NFPA 70E recommendations is the most prudent course of action. For best practice use an arc ear plug or a silicone plug but assure that you have adequate protection for the hazard over all exposed skin and you should never have an issue with an ear canal insert.
See the slide from e-Hazard.com training below for examples of the plugs we tested (we did 100 ear plugs to determine the probability of ignition for the plugs. Foam always melted but the yellow we tested did not ignite.) ArcWear.com and e-Hazard.com do not recommend specific brands or sell PPE. We provide this testing as a service to the industry for training purposes. The testing is copyrighted by ArcWear.com and may only be used with permission.