I'm using Emacs 27.1 on Debian Stable, using tmr 0.3.0 from GNU Elpa. When I set a timer with a sound file defined, I get this error when the timer ends:
TMR Time is up!
Error running timer ‘tmr--complete’: (invalid-function (sound tmr-sound-file))
I'm still learning ELisp and I'm not able to do much in the way of useful debugging. I did note that I do have ffmpeg installed (ffplay exists and is available in $PATH).