The problem which you see is most probably due to a too low amplitude of at least one of the RF signals fed into the module which gives a too low AF amplitude at its output. The latter makes that the audio fades out when one of the two knobs is turned too far clockwise.
If you own an oscilloscope, I can guide you through the diagnose which will tell us if something happened during the installation of the module or if the module itself has a problem. If you don't have an oscilloscope, I invite you herewith to send me your Etherwave PCB with the module installed in, so that I can diagnose it and fix everything. In ever case, I am here to help you, so that you will have full benefit from the module. Best is to contact me by email thierry(at)theremin(dot)tf so that we might discuss the further proceeding.