Simplify process to add & play sound

[nighca]

Present situation

To add & play sound in Builder, you need to:

1. Add sound (upload/record/choose) with name foo

2. Define sound vars in the code of Stage

   var (
     foo Sound
   )

3. Call play foo in code

Step 2 may be avoidable, & step 3 may be simplified

Expected

1. Add sound (upload/record/choose) with name foo
2. Call play "foo" in code