Why Im getting "sox.exe FAIL" ?
This error may appear when you are trying to mix two different audio files format (44,1 kHz, 48khz, 96kHz).
Yes but why? Well, Think of it like trying to combine 2 different file formats like PDF+JPEG for example.
In order to fix this issue, Please make sure that your files have the same Sample rate and bit rate for both tracks and watermark files.(if your original tracks are WAV 48KHz so the watermark files should be also the same sample rate etc..