A Hydra's Standard Action attack may attack once for each head, but Martial Strikes don't. They all (IIRC) specifically say "make a single melee attack" - and that's what you can do. You used your Standard Action to do it, so your other heads do nothing. Strikes do not activate on attacks (or else you could use four during a Full Attack), nor do they activate your normal Standard Action attack routine, they specifically allow you only to attack once. Snap Kick, IIRC, gives an explicit exception to this, but other than that, no.