What would we have seen in society and the life of Southeast Asia before 1500 AD?

The main thing that we would notice about life and society in Southeast Asia before 1500 AD (so long as we do not go back too far) is that strong influence on the region from both China and India.  Southeast Asia was located close to both of these powerful and influential empires and was affected by both of them.

As one example of this, we would see the influence of Hinduism and Buddhism coming in from India.  The Funan Empire in the Mekong Delta area controlled trade between India and China early in the Common Era.  The people who actually did the trading, though, were mostly Indians.  Buddhist monks and Hindu priests came along with the traders and exposed the Funan Empire and its surroundings to their faiths.  We could see this also in the later Khmer kingdom.  We could see how its religion (as seen at Angkor Wat) was influenced by the coming of Indians.

Later on, we would have seen Chinese influence in what is now Vietnam.  We would have seen Mahayana Buddhism coming from China along with Daoism and Confucianism to strongly influence Vietnamese religion and society.  We could also see Chinese language and writing being used in Vietnam.

Thus, much of what we would have seen in Southeast Asia at this time would have reflected the influence of China and India.