There is good news and bad news. The good news is when working with just an external sound/PA system via the headphone output is you get to hear what the audience is hearing, thereby allowing you to set the EQs and volume level on the fly if something doesn't sound just right.
The bad news is if you were able to use both the internal sound system and an external system at the same time, it is likely that if the external system were to go awry for some reason, you would not know it because you were concentrating on those internal speakers, but you can bet your bottom dollar that the audiences and ADs would hear it.
The simple solution is to purchase a vertical array, external, sound system, such as the Bose Compact, which provides outstanding sound quality, a 210 degree sound array, and you hear exactly what the audience hears. No monitor needed! Yes, the Bose is expensive, but it is well worth the expense.
Good luck,
Gary