Adjust the 'fade' value (i.e.\ 'balance' between front and rear)
* for the specified volume with the specified channel map. v will be
* modified in place and returned. The balance is a value between
* -1.0f and +1.0f. This operation might not be reversible! Also,
* after this call pa_cvolume_get_fade() is not guaranteed to actually
* return the requested fade value (e.g. when the input volume was
* zero anyway for all channels). If no fade value is applicable to
* this channel map the volume will not be modified. See
* pa_channel_map_can_fade(). Will return NULL on error. \since 0.9.15
Adjust the 'fade' value (i.e.\ 'balance' between front and rear) * for the specified volume with the specified channel map. v will be * modified in place and returned. The balance is a value between * -1.0f and +1.0f. This operation might not be reversible! Also, * after this call pa_cvolume_get_fade() is not guaranteed to actually * return the requested fade value (e.g. when the input volume was * zero anyway for all channels). If no fade value is applicable to * this channel map the volume will not be modified. See * pa_channel_map_can_fade(). Will return NULL on error. \since 0.9.15