I discovered a solution to this low volume issue different from the above posts.
My Surface Pro 3 has all the current updates. I started to experience speaker volume issues several months ago. Depending on the audio source sometimes the volume was fine, but other times I could barely hear anything. For example, the Netflix "app" worked fine, but Netflix on a browser had barely audible volume. But for most audio programs the volume was so low I could barely hear it. I checked all the volume controls and nothing seemed to help. But then, while I was poking around to see what could be the issue, I discovered the following:
I have Google Chrome installed. The standard mode for Chrome is a browser on the Windows Desktop. But, if you pick the settings icon (upper right), at the bottom of the pop-up list is an option to "Re-launch Chrome in Windows 8 mode". This makes Chrome full screen with a row of Google icons at the bottom left of the screen. But...on the right side at bottom is a digital clock. If you pick the clock a pop-up appears with a volume slider! This volume control was set to "zero". When I slide it to the right (full volume) it solved my volume issue for all other program volume issues! I don't think this volume control is accessible from the standard desktop Chrome browser mode.
Anyway my volume issue is solved...!