rotate files written by voctomix more often

The files written by voctomix currently rotate every 30 min. The partial files are not usable. Rotating them more often ensures that the files data is on disk in a usable format more quickly.

If we could find a way to write data to disk in a format that allow the partial files to be useful, that would be even better.

ffmpeg can be told to put the necessary info at the beginning of the file, but it's probably only written there when the file is closed, so this might not be useful in our case.