when i play parcel music,how can i use script to change the volume

I have made a hifi in second life. when i click it , there will be a menu to choose songs. But i want to make a button to change the volume of the music by lsl. how can I do that? Thanks a lot

You can't. There's no such LSL function.

I suppose that if you absolutely positively needed this functionality, you might be able to get it for just Restrained Life viewers, but I think you'd also have to add some functionality to RLV itself. The marginal utility of such an endeavor, however, would seem vanishingly close to zero.