From QB64 Wiki
The _SNDVOL statement sets the volume of a sound loaded in memory using a handle from the _SNDOPEN function.
- _SNDVOL handle&, volume!
- volume! is a value from 0 (silence) to 1 (full volume).
- In versions prior to build 20170811/60, the sound identified by handle& must have been opened using the "VOL" capability to use this function.