Set the volume for the music channel.
int Mix_VolumeMusic(int volume);
the new volume, between 0 and MIX_MAX_VOLUME, or -1 to query.
Returns the previous volume. If the specified volume is -1, this returns the current volume.
The volume must be between 0 (silence) and MIX_MAX_VOLUME (full volume). Note that MIX_MAX_VOLUME is 128. Values greater than MIX_MAX_VOLUME are clamped to MIX_MAX_VOLUME.
Specifying a negative volume will not change the current volume; as such, this can be used to query the current volume without making changes, as this function returns the previous (in this case, still-current) value.
The default volume for music is MIX_MAX_VOLUME (no attenuation).
This function is available since SDL_mixer 2.0.0.