One of the worst user experiences I have experienced in the recent years is those notifications telling me that my volume is too loud. But such assumption most of the time is totally wrong as it is hardware dependent. Especially when you use additional speakers or headphones with their own volume regulators. On windows 8.1, every time you make your volume higher than 40%, you will be interrupted with notification about high volume causing hearing loss, similar tactics is used in android.

So in my opinion checking system volume to determine whether the sound is too low or too high is completely wrong approach.

I use an external amplifier connected through a front-panel IO port. Both the front panel and my amplifier have volume knobs. I don't use the system try to control my levels. I know I'm an odd case, but surely there many people out there with basic integrated speaker setups with volume knobs.

The simplest solution would be to visually indicate to your user as early as possible that your page uses audio. If it is designed to be particularly loud or quiet, you can mention that, too.