How is it possible to change in real-time the start and stop position of a soundfile in loop mode without getting audible audio clicks.

other options are:

(1) use 2 loop players and crossfade between then every time you change the position.  (you can put each player in a separate subpatch and use [switch~] to turn it off after the crossfade is finished, to save on processing power)

or (2) use [vline~] to  fade your audio out before the position change, and back in after it. 
so, something like: 

[r newposition]
|
[pipe 5]
|
[looperplayerthingy~]
|
|      [r newposition]
|      |
|      [t b]
|      |
|      [0 5, 1 5 7]
|      |
|      [vline~]
|      |
[*~ ]

(3) you could also use a voltage controlled Lowpass filter instead of the simple volume fade to do a similar thing to (2), but better.  this would either require an external, or making a filter yourself using [czero~] and [cpole~] or whatever.