Dog pissed on speaker

A bit fussy to do, but an electric varmint fence unit plus a moist pad he'd have to stand on to pee would be a very real disincentive.  Next time, a small female dog might be a better choice.

The easy way would be to get a couple of 'scat mats' from Amazon and put them in front of the speakers. They come in two forms - one just makes loud shrill noises if an animal steps on them and the other actually delivers a shock to them.

Search "electric no pee mat for dog" on amazon

Maybe your dog can hear better than you and he is trying to tell you something - get rid of the speakers. 

Seriously here's what you need to do. Get a bowl of warm water and add a tablespoon of Arm & Hammer baking soda, stir it good then dampen a cloth with this solution, ring it out then gently wipe the area where the pee is. This will neutralize the pee (ammonia). The wipe again with clean water. Don't over do it.

Maddening, isn't it. Flipping cats(four)and dogs(three)like to go in the front room where I have my ping pong table. 

Only one person suggest an ozone generator.  Least likely to damage the components. That would eventually break down the uric acid, urea, and other nittrogenous compounds generating the ammoniacal smell after bacteria and enzyme exposure. UV light (germicidal) might work, but don't expose paper to it (no speaker cones) only the box.