The VideoLAN Forums

In older version, setting --gain=6.0 boosted the volume to 600% without distortion.
In recent version of libvlc, --gain=6.0 distorted the sound while --gain<1.0 is OK.

Gains above 1 have always been potentially subject to saturation depending on source format and output. Gain is meant to compensate for source recording at too low volume, not for general amplification.

And besides there are no supported means to control gain via LIBVLC so you're on your own.