It's a known issue with stb emu / Smart STB on Sony smart tv
Long press ok/select button and click settings
Click key configuration settings
Next click grey bar that says 'add key'
Press your chosen button (we used fast forward)
Go down to 'select action' and click on it. Then scroll down to 'volume down' and select it.
Then click on 'save' below it.
Go back out, select channel, turn it down and fingers crossed you're done!