Hi, I don't really get the optional arguments to the open message (after
the filename)
Onset is basically where to start playing but counted in samples from the beginning. If you know the sample rate of the file, you can calculate the onset for a given time, say 1 second in and start paying for there.
The rest are sound file header overrides to force how the format of the file in interpreted, ie. you can force readsf~ to play a stereo file in mono for instance or if you have some sort of raw output which is missing some encoding you can force the format, etc. Those values are similar to the sound filer -raw flag values.
and I don't understand how the mono file input (bell.aif) can
output 4 channels of audio!
It doesn't... there are 4 output channels from the object but only the first shows a change in envelope when you play it. The rest output 0 as the file is only mono. This just means the the object is created to play *up to* 4 channel files but that of course depends on the files it gets.