The S2415H is a Stereo two channel (L/R) output only. When using the S2415H built-in 3w speakers, a 4 or higher channel audio setup will not work. So the first thing to do is make sure that the PC audio is set to Stereo.
* Press the S2415H Menu button to open the OSD (On Screen Display)
* Go to Audio
* Go to Audio Source and choose HDMI (MHL)
* Set the volume to 50
* Exit the S2415H OSD
* Open the computer audio Sound Playback and click the DELL S2415H HDMI as the default playback device. This places a green check on the S2415H HDMI listing. See my example below =
From the computer green audio out jack, plug an RCA cable in it, the other end plugs into the S2415H audio Line-In port. Set the computer audio playback to Speakers Realtek. Test both the S2415H speakers and its headphone out jack.
I would also test the S2415H on another computer to see if the "no audio" issue follows the S2415H.