I just tried some more. I am sure that last time I did it it was that file. Maybe it's that in conjunction with another. I just made some changes to the dictionary, and after quitting the program I saw that there was another file which updated. It has a 16 character file name (hex digits).db
The customisations that I just made were in there. Now, I suspect that the sixteen digit file name changes with each install, so if you have a previous file from an earlier installation you may have to rename it to the name of the equivalent file in the new installation. Only trouble is that file wasn't there in my previous install - I could have sworn that I had customised some of the speech in the last installation. Anyway, I have now, and I can at least save those changes and then get them back if I screw things up. Only time will tell whether that works across an installation though. If you have such a file in your back up, and if PMM creates a new .db file with a 16 digit name, try renaming the old file to the new name and then copying it over.
I also just did some tests with voice_dictionary.db It seems that PMM uses this file to store the details of the voices used. If you delete the file and start up PMM, all entries in the dictionary go to the default voice, but only if the other db file was not already present. If you specify some different voices, they go in that file I think.
I'm sorry that the previous information was not complete / misleading. I guess we're all still learning stuff.