The Selective Mixing of Sounds is a large procedure, a large part of which is conceptually challenging and has not been developed previously. This is a method of determining areas in the time-frequency plane. It has a major effect on the overall quality of the sound. In this paper we propose and compare a range of appropriate algorithms. We begin with a simple two-dimensional running mean combined with a rule selecting the track characterised by the maximum energy, followed by a low-pass filter based on the 2-dimensional Fourier transform. We also propose two novel methods based on the Monte-Carlo approach, in which local probabilistic rules are iterated many times to produce the required level of smoothing.