Windows 7, how to enable both sounds?

Greetings from long-time Dell User!

I wonder if I may enable both sounds from external speakers and the built-in speakers of the Dell S2415H? I try to configure the sound settings but Windows 7 just let me enable either the external speakers or S2415H.

Model of sounds system from my laptop: Lenovo T420
1/ 2 in 1 Soundmax computer-standard speakers, sound out through 3.5 jack
2/ Built-in speakers of monitor Dell S2415H, sound out from Display port of T420 to HDMI cable of the monitor; this is connected by the Display port to HDMI converter.

Thanks,