If the mixer output is being played, there will be some delay in the effect of adding/removing source channels or changing their attributes being heard, due to buffering. This latency can be reduced via the BASS_ATTRIB_BUFFER attribute. The playback buffer can be flushed by calling BASS_ChannelPlay (restart = TRUE) or BASS_ChannelSetPosition (pos = 0). That can also be done to restart a mixer that has ended.
Does this apply to a decoding mixer?