If your situation isn't so severe (or you are generally happy with the volume), there are a couple of things you can do to give yourself greater control over the volume. Part of what might be causing the trouble is that any number of apps or players on your Fire can access and change the volume depending on what is running. There are also a handful of different volume levels for the Fire, just like your computer. It may really only be one of them that has changed, and so adjusting the master volume may not make much of a difference.
The solution? You may be shocked when I tell you that there's another app to help you handle this. There's an app for everything, isn't there? Actually, when it comes to the volume there are several, a couple that are free and add supported and a couple that cost a buck or two and have extra features but no ads. Let's take a look.
Volume Manager Free Volume Master Free
Volume Manager Pro Volume Master Pro
There's also the QuickMute Volume Control to quickly turn off all sound with one click. The above apps will give you access to all of your devices volume controls, allowing you to individually adjust them. The paid ones let you make different profiles for sound levels, and if that's valuable to you it may be worth paying for them. Otherwise, as long as you don't find the ads intrusive, you might be fine with the free versions.
Take a look, pick an app that you think will work best for you, and then you'll be able to reliably adjust your sound volume and hopefully not have any problem with it being low or changing suddenly.