Please How to use code fences
Create a Number or Dimmer Item to represent the state of your volume. Put this on your sitemap using a Slider element.
Slider item=Volume_Proxy minValue=0 maxValue=100 step=5
When the number changes, you need to issue the appropraite number of VolumeUP and VolumeDown commands to the Items linked to the device.
However, where this is all going to break down is if you do not have any feedback from the device telling you what the actual volume is, you will likely end up getting out of sync between your proxy Item and the actual volume of the device, especially if it’s possible to control the volume outside of OH.