This sounds like a known bug in wingpanel-sound-indicator. I recommend marking that bug as 'affecting me' or opening another on the bug tracker if yours is in fact different.
In the meantime you could try remapping your stereo output to mono by following one of these examples.
App badges on Plank cannot be enabled or disabled — they simply appear when apps that have implemented support for Plank app badges. Unfortunately, not every application on Linux has implemented this integration. Thus, the only thing you can do is be sure you are using an application which supports that feature. These include:
I'm pretty sure you've linked to the right answer: Plank's badges are separate from notification center, and they have to be implemented separately. However, you might consider adding a wishlist item on the launchpad.