Multi-step method for producing a soundproof composite cover for internal combustion engines, suitable to produce a composite cover made of thermoplastic material and insulating foam with a predetermined shape so as to enable the complete covering and desired sound insulation of the aforementioned engine; said method making use of a multiple mold made up of a first cell, provided with a male element (101.a) and a female element (101.b), and at least one second cell, provided with a male element (102.a) and a female element (102.b), said first and said at least one second cell (101, 102) being suitable to take, in a simultaneous and reversible manner, a first closed configuration wherein said male elements (101.a, 102.a) and said female elements (101.b, 102.b) are partly in contact with each other, respectively through a first contact area (101.c) and a second contact area (102.c), so as to respectively define a first interspace (101.d) suitable to be filled with the thermoplastic material and at least one second interspace (102.d) suitable to be filled with the solidified thermoplastic material and with the insulating foam; said cells (101, 102) of said multiple mold also being suitable to take, in a simultaneous and reversible manner, a second open configuration wherein said male elements (101.a, 102.a) and said female elements (101.b, 102.b) are mutually spaced to enable the extraction of the finished product; said male element (102.a) of said second cell of said multiple mold being characterized by at least one punch, preferably metallic, suitable to make said second cell hermetic when said multiple mold is in said first closed configuration, so as to contain the insulating foam during the expansion thereof between a foaming step and an extraction step; said method also being constituted by the following subsequent step s: A) Injection step: in said injection step said multiple mold is in said first closed configuration and, through a channel arranged at said female element (101.b) of said first cell, a predetermined amount of thermoplastic material is injected from a hot chamber to said first interspace (101.d) of said first cell; B) Transfer step: after a predetermined period of time starting from the end of said injection step, said thermoplastic material cools and reaches the solid state so as to enable, in said transfer step, the opening of said multiple mold in said second open configuration to extract the solidified thermoplastic material from said first cell and position it in said second interspace (102.d) of said second cell; C) Foaming step: in said foaming step said multiple mold is closed and once again takes said first closed configuration and, while a new injection process occurs in said first cell, a predetermined amount of insulating foam is injected into said second cell in the portion of interspace (102.d) present between said solidified thermoplastic material and said male element (102.a), so that, by solidifying, said insulating foam irreversibly adheres to the solidified thermoplastic material with which it comes into contact; D) Extraction step: after a predetermined period of time starting from the end of said foaming step, so as to enable the solidification of said insulating foam, said multiple mold is opened and once again takes said second open configuration; in said extraction step, the finished product is extracted from said second cell, and simultaneously the solidified thermoplastic material is extracted from said first cell and arranged in said second cell thus leading to a new transfer step. A) Injection step: in said injection step said multiple mold is in said first closed configuration and, through a channel arranged at said female element (101.b) of said first cell, a predetermined amount of thermoplastic material is injected from a hot chamber to said first interspace (101.d) of said first cell; B) Transfer step: after a predetermined period of time starting from the end of said injection step, said thermoplastic material cools and reaches the solid state so as to enable, in said transfer step, the opening of said multiple mold in said second open configuration to extract the solidified thermoplastic material from said first cell and position it in said second interspace (102.d) of said second cell; C) Foaming step: in said foaming step said multiple mold is closed and once again takes said first closed configuration and, while a new injection process occurs in said first cell, a predetermined amount of insulating foam is injected into said second cell in the portion of interspace (102.d) present between said solidified thermoplastic material and said male element (102.a), so that, by solidifying, said insulating foam irreversibly adheres to the solidified thermoplastic material with which it comes into contact; D) Extraction step: after a predetermined period of time starting from the end of said foaming step, so as to enable the solidification of said insulating foam, said multiple mold is opened and once again takes said second open configuration; in said extraction step, the finished product is extracted from said second cell, and simultaneously the solidified thermoplastic material is extracted from said first cell and arranged in said second cell thus leading to a new transfer step.