remove a useless line

 because

  `num get _volume => _sourceNode.gain.value;`

the removed line can cause "Null Error" and hide the root cause. (in my case on Firefox, now,  I have : 

[16:01:14.338] "audio	ERROR	[Exception... "An attempt was made to use an object that is not, or is no longer, usable"  code: "11" nsresult: "0x8053000b (InvalidStateError)"  location: "http://127.0.0.1:3030/vdrones/web/index.dart.js Line: 15521"]" 

)