Caution: I’m not an expert in the area of film music so don’t really know which are the great journals or if any are particularly dodgy (eg journals participating in predatory open access publishing) so beware.
• Music and the Moving Image http://www.press.uillinois.edu/journals/mmi.html
• Music, Sound and the Moving Image http://muse.jhu.edu/journals/music_sound_and_the_moving_image/
• The Journal of Film Music https://www.equinoxpub.com/journals/index.php/JFM
• Music and moving pictures (mailing list) https://www.jiscmail.ac.uk/cgi-bin/webadmin?A0=MUSIC-AND-MOVING-PICTURES