Q: If I reveal Sonic Boom, and all my characters are already exhausted, can I still choose the option to exhaust all characters I control?
A: No. If possible, you would have to spend the resources, if all the characters you control are already exhausted. Sonic Boom is a choice, but you must choose an option that you can fulfill. If you cannot fulfill either option, then you must do as much as you can, which typically means discarding one or two different resource icons from your hand. -(Developer Ruling, Hall of Heroes)