Can an external mic. be used instead of a headset or will the sound from speakers cause interference with mic?
It depends if it's voice partially, but really sound is measured in specific wavelengths, it tries to match the pattern to specific general wave lengths, if matches, execute command.
The louder the audio is, the more it's going to distort that wave length and make it not match. So depending on volume settings, or how loud you are personally is going to decide if it's viable for you or not.
I definitely recommend head phones for a more immersive experience.