They are an item that you have to place in your inventory. Once you have them in your inventory, use your ACE self interaction key to bring up the menu. Find "equipment" and you should see the option "earplugs in".
Ok I haven't got a clue... Dumb it down for me. Step by step
Go to an equipment box. Open Arsenal. Select one of your three pieces of clothing (uniform, vest, or backpack). In the upper right corner, click the black box with the + in it. One of the pieces of equipment in the adjacent list will be Ear Plugs. Click the Ear Plugs and the little + to the right of the text (not the same + you clicked to get to the menu) to add one to your inventory.
Exit Arsenal.
Press Win+Ctrl to open the self-interact menu. Mouse to Equipment -> Ear Plugs.
Your overall volume level will reduce by 1 or 2 notches (because your character is now wearing ear plugs, naturally), but the ringing effect from very loud weapons will stop.
