No sound from display connected with HDMI

I just bought a monitor (LG 29WL500) for my MacBook Pro (Retina, 13-inch, Early 2015) until I can fix its broken screen!

The video works fine but I have no sound from the monitor's built-in speakers. I used the HDMI cable that came with the monitor.

I've already tried changing the sound output in System Preferences > Sound > Output but still no sound. Also, on the settings instead of the balance controls it says "The selected device has no output controls".

Can anyone help me?

Thanks in advance,

Alex