My Linux Mint install has had a weird problem lately where the volume control main sound slider didn't affect the volume of anything noticeable. The Applications sound slider control did affect the volume on a one-by-one basis. After getting frustrated and starting to click things a little more randomly than I usually would,  I switched to the HDMI hardware output instead of the normal built in analog, and sound stopped, but when I switched it back, the master volume slider started suddenly working.