This application for Sony SmartWatch allows you to modify the music volume on your phone.
Press on + to increase or - to reduce the volume. The bar indicates the current value.
If you want the widget version, take a look at "VolumeWidget - Smart Extras™" (the touch area is smaller in the widget and less responsive).
Icons are from IconEden (http://www.iconeden.com)
LiveWare™ extension for SmartWatch 1 (not tested on SmartWatch 2)