Monitor speakers are not set up correctly.
The Dell U3415W monitor has two nine watt 100 Hz to 20 kHz integrated stereo speakers (Figure 1).
Figure 1
The U3415W has multiple audio input options (DisplayPort (DP), Mini-DisplayPort, HDMI, and MHL-HDMI). If you are planning to use the internal monitor speakers, then you must adjust the audio output selection on your computer or the audio input configuration in the On Screen Display (OSD) of the monitor.
The monitor has a volume control in the OSD apart from your computer having a volume control. The suggested initial display volume setting is 50% (Figure 2).
Figure 2
The OSD has audio input options that you can customize for your configuration (Figure 3).
Figure 3
The U3415W also has audio out allowing you to connect speakers to the monitor.
To configure your computer volume, Right-Click the Speaker Icon on your taskbar, select the Playback Devices option, then select Dell U3415W as the default playback device.