Set different volume for multiple apps running at the same time I want to use Spotify and a running speed tracker app at the same time, but Spotify is much louder than the other app and so I cannot hear it when both running simultaneously. Is there a way I can turn the volume down on one app but not the other, similar to volume mixer in Windows?
Probably you were looking for something like Sound Assistant version, for generic Android devices, but as it stands now the former is available and its only functional on Samsung devices (Galaxy editions running latest versions of Android)

From description:

SoundAssistant


  The sound utility available on the galaxy series Provides various
  convenience functions related to audio
  - Allow you to control the media volume instead of ringtone when pressing the volume keys
  - Support individual application volume (You can control the volume of music and games differently)




Unfortunately as it stands it is not possible or at least feasable for other android devices, perhaps this feature will need some hardware upgrade on target devices to stream audio output on more than 1 channel, just as Izzy commented in his answer. 

You may have to wait a little while until such a feature can be impelemented in other android devices, meanwhile you could get a supported Samsung device to enjoy this feature or perhaps automation with tasker or macrodroid.