Download and extract FFmpeg
Open a new command prompt as administrator and change directories to the ffmpeg bin folder.
Place your decrpyted video and decrypted audio files in this bin folder as well
In command prompt issue the command to merge the two streams
ffmpeg.exe -i INPUTVIDEO -i INPUTAUDIO -vcodec copy -acodec copy FINAL
INPUTVIDEOis your video file,
INPUTAUDIOis your audio file and
FINALis your merged Audio/Video file.
Congrats, you now have a playable decrypted widevine audio/video file!