The strictest way to do it is with this regex:
^(.*?)\d{4}-\d{2}-\d{4}_\d{2}_\d{2}_\d{6}\.mp3
The result will be in the first capturing group.
The regex only assumes the format of the date is fixed. It will work correctly even if the name contains some numbers.
Alternatively, if you can assume that the format of the date time is consistent, you can just do a substring that takes from (0, length - 29).