If your site features audio of someone speaking, you need to make sure any background noise on the track does not drown out the speech in the foreground, as those who are hard of hearing may have a difficult time understanding the clip.
Thankfully, the fixes for this one are pretty straightforward. The easiest solution, of course, is to select audio tracks that have no background noise
If you are using audio with a lot of background noise, though, try opening the file in an audio editor like Audacity or Garageband to remix the track. Try using a parametric EQ to discern the frequency ranges which are particularly troublesome, and then bring those frequencies down while raising the gain on the frequencies encompassing the bulk of the speech you are trying to isolate.
Let's take a listen to this clip of David Sedaris reading a story for This American Life.
You'll notice that there is a background track underneath his speaking, but it is low enough in relation to the volume of his voice that it does not pose an issue.
Another way to satisfy this rule is by including a mechanism on your site that allows users to lower or mute the background noise themselves.
There are a few exceptions to this guideline. You don't need to worry about the background volume if your audio clip is one of the following: