I found this info for you. On my recorder, these files are small, and probably do not contain the actual recordings, only data about the recordings. From Sony website:
What is the MSGLIST.MSF file?
Applicable Models:
PCM-M10, PCM-D50
Article ID : 1187263 (modified 11/16/2009 11:54:00)
Question:
About MSGLIST.MSF file.
Answer:
MSGLIST.MSF is a file that contains information such as the order of folders, playback order of audio files and recording date and time. If user connects the recorder to the computer, the MSGLIST.MSF files are displayed but do not delete or change these files. If these files are modifed, the saved files may be broken or the recorder may not work correctly.
Hope this helps.
ChgoDoc