Thank you for the hints. This worked for me when the FFProbe output didn't redirect to a file.

It's A slight Variation of the solution above. Don't need sexagesimal for adjusting -itsoffset .srt, just the seconds, so good to go using absolute path of ffprobe location and the batch variables to indicate path of .mp3 and MP3_Duration.txt

@echo off

cmd /c C:\FFMpeg\Bin\ffprobe -loglevel error -select_streams a -show_entries stream=duration -of default=noprint_wrappers=1:nokey=1 -i "%~d1%~p1%~n1.mp3" > "%~d1%~p1MP3_Duration.txt"