Many third-party voice memo or call recording apps use this naming convention for active recordings.
Apps like GarageBand or Audacity may create .tmp or RecordTMP files while you are recording to ensure data isn't lost if the program closes unexpectedly. How to Access or Fix the File RecordTMP (32) m4a
If the file won't play, it is likely missing its "header" (the data that tells a player how long the audio is). You can try these methods: Many third-party voice memo or call recording apps