UPDATE: With @toggledbits update for "R4V" below, your new notification sounds will work just by typing your custom soundfile name into the Sound: field of the Notify Action. Editing the J_ReactorSensor_UI7.js is not an undertaking for the simple user 
Please ignore the entire "Tip" below, it ended up hosing my Reactor interface and I had to re-install the Reactor "stable" version to get back up and running. I'll leave it posted for a bit in case someone knows WHY it got hosed.
Sorry,
Chris
Guessing this is the most appropriate area in which to post this . . .
In case nobody has written this yet, I've been using Pushover for a while. First with VeraAlerts on Vera3/UI5, now with Reactor on openLuup/RPi3.
Until recently, Pushover has constrained us by only allowing select notification sounds. Now pushover.net will allow you to upload custom sounds to their internal library.

You may then add these sounds to Patrick's J_ReactorSensor_UI7.js file
values: [
"=(device default)", "none=(none/silent)", "vibrate=(vibrate only)", "pushover=Pushover",
"bike=Bike", "Bubbles=Bubbles (VA)", "bugle=Bugle", "cashregister=Cash Register", "classical=Classical", "Cloud=Cloud (VA)", "cosmic=Cosmic", "falling=Falling",
"gamelan=Gamelan", "incoming=Incoming", "intermission=Intermission", "iPhone_Ding=iPhone Ding (VA)", "magic=Magic", "mechanical=Mechanical",
"pianobar=Piano Bar", "siren=Siren", "spacealarm=Space Alarm", "tugboat=Tug Boat", "alien=Alien Alarm (long)",
"climb=Climb (long)", "persistent=Persistent (long)", "echo=Pushover Echo (long)", "updown=Up Down (long)"
Not sure how the above code will format here, but I searched for "Pushover" in the .js file, found the section above, and sprinkled my 3 custom sounds alphabetically into the existing group (following existing formatting)
After that, a browser refresh was all that was needed.
That's All Folks,
Don't Call Me Chrissy