Please search the Minim documentation - all of these answers are there.

http://code.compartmental.net/tools/minim/ ... found from http://processing.org/reference/libraries/

#1

You can set whether its stereo or mono. You access the buffers of either channel, or a mix of them, by bufferSize() and using left.get() , right.get(), or mix.get() respectively. See the getLineIn example.

#2

Your current millisecond of the 'playhead' can be gotten by the position() method, and the total length by the - you guessed it - length() method.

#3      

There is a boolean isPlaying() that will return true or false depending upon, well, whether the sample is done or not. 

This is assuming that the sample is not set to loop.

I need a further explanation:

how can I check if a part of a sound that has made ​​a beat.isKick() is finished?

How can I use the method that you explain in #1 with an AudioPlayer?