Monitor without integrated speakers / with PS3 sound through headphones?

I would like to play Call of Duty BO2 on PS3. How do I get sound on my headphones? It's very simple on the PS4.

I've already bought an adapter, but I can't hear more than a hiss in my headphones.

The PS3 is connected to the monitor via HDMI and the headphones to the PS3 via a USB adapter.

Don't you have boxes

Do sometimes connect speakers to at least know whether it is on the PS3 or something else.