The BASS_Mixer_StreamAddChannelEx "start" value is based on the mixer's sample format. For example, if you wanted to delay the start of a source by 5 seconds, you could do this...
QWORD start=BASS_ChannelSeconds2Bytes(mixer, 5); // convert 5 seconds to bytes
BASS_Mixer_StreamAddChannelEx(mixer, source, 0, start, 0); // add the source to the mix