Unusually low volume?

[AC Question]

Recently I have been having problems with the volume on my phone (Samsung Galaxy S5, if that matters) when listening to music. Even when it's on max, it's still pretty quiet and it bothers me a lot. A few times after plugging my headphones in I could hear the volume the way it should be but after about 10 seconds it gets much quieter. I downloaded a volume booster app but it only works once in a while and all the other ones I've tried don't seem to work. I don't think the headphones are the problem, either. Does anybody know how to fix this? Thanks in advance!