What file format does the Voice Recorder output? If it is something common, the SoX program I mentioned should be able to do it.

Syntax : sox old.wav new.wav trim [SECOND TO START] [SECONDS DURATION]

For a 90 minute recording:

sox old.wav new.wav trim 1800 3600

EDIT: For the sake of clarity, I have to mention that you would use this command AFTER you have already recorded with your voice recorder app. I'm assuming you would have your voice recorder app running constantly. After you stop recording you would use the sox command I described with the audio file produced by your voice recorder app. The example I used was for a 90 minute recording, but if you have a file of indeterminate length, "Sox [filename] -e stat" will give you the length of the filename in seconds. This number in seconds minus 3600 seconds is your first parameter after "trim" in the command above. And since you want one hour, the last parameter is 3600. So if the length of the audio file were 4689 seconds the last hour of audio would start at the 1089 second mark. So the command would be

sox [output file from voice recorder app] new.wav trim 1089 3600.

Hope that is clear.