Not with MX that I can see, even the key map manager does not claim support for TC51: Keymapping Manager - Zebra Technologies Techdocs
If you have control of the applications the standard way to do this is for the application in question to set the audio stream being controlled by the volume buttons: https://developer.android.com/guide/topics/media-apps/volume-and-earphones.html . There are also 3rd party applications that claim to do this configurably in a service, e.g.: Rocker Locker - Android Apps on Google Play but surely the next application to come along could just switch the volume behaviour back unless the service continually sets the volume stream which does not seem right... anyway, there is nothing official from Zebra that I am aware of for this.
All the new Marshmallow devices (TC51/TC56 and TC70x/TC75x) support Mx Keymapping Manager - Zebra Technologies Techdocs
You can make a quick test with StageNow, building a couple of Barcodes to apply the configuration you want and to re-apply the defaul configuration.