You may not be able to control the volume, but you can mute it from the handlebars the same way you normally would mute music.
I don't have any problems with it not being loud enough though. definitely make sure the speakers are directly over your ear. cut out some foam in the helmet if you have to.
I do wish it would turn on and off automatically with the bike though. If you don't turn it off the battery in the sena dies and has to get some charge before it will work again.