If you recall, Pain stated that Itachi was able to infiltrate Konoha whenever he wanted as a result of his experience in the ANBU. Meaning Itachi had access to the Sensing Barrier pass. As a result, Orochimaru, who was also in the ANBU, had the same advantages. So, it would only stand to reason that the reason the Sound Four could infiltrate Konoha so often, was because of Orochimaru giving them the pass, as well as Konoha's own security issues following the Konoha Crush.