Is it possible to reset Engineering mode's audio settings?
Open engineering mode using the number *#*#3646633#*#*
Then go to hardware testing
Press audio
Then go to loudspeaker mode
Press ring change to media 
Change value 0~255 change to 60.


I.e...., from if you have messed up everything in all the setting  sid, sip, ring, sph and sph2
From level 0 to 6or 14 the value 0~255 should be in increasing order in counts of 12 or 16.
And value 0~160 keep that in 128 default or for increase in volume keep in 135 that's all.

Do this properly and you will get the slider corrected and increase in volume. 
While doing this process play a song from the music player. 

Even I faced the same issue and got it solved after an hour of experimenting with all 4 audio settings
Don't alter engineering mode again.