The best way to detect whether all urine residual is no longer present is to first smell, as you have been doing and then run your tongue over the Speaker Cone and surrounding Cabinetry.
If you feel confident there is a taste then I would take this as an surety that the dog might be due a return boundary marking exercise.
It looks like the dog is looking to be Alpha and possess the items the Alpha shows interests in.
Be aware with this developing the dog might just confront you aggressively if you attempt to use their newly possessed Audio System.