I just created a free chrome extension for low time alerts.
It has:
- Custom low time notifications with a few different sounds
- Optional 5 second countdown
- Optional screen flash if you're hard of hearing
You can find it here: https://chromewebstore.google.com/detail/chesscom-low-time-alerts/ndfkkhkkmekhknheeoecnkogaodjecmd
Hopefully this is helpful for you 
I play rapid 10/5 mostly these days and I think it would be preferable if I could adjust my low time warning to 5 or 10 seconds. 1 minute doesn't help especially when my time remaining hovers around 10 seconds when I've used up all my time and getting 5 seconds back each move.
If anyone knows how I can do this for rapid that would be great. If not, I think it would be handy for chess.com