Universal Media Server | Forum

A good indicator of when a file is being transcoded is higher CPU usage, but how obvious that is depends on your system. If it is a modern powerful computer you might not see as much difference compared to an older one with only a few cores then it will be easier to tell.

If you turn on "add engine names after filenames" in the navigation tab then you will be able to see what files will be transcoded if played as it will show (ffmpeg) for example added to the end of the filename.

It would be possible to just transcode the audio and remux it, but this is not implemented in UMS as far as I know. As to the CPU uses you provide, I have no good explanation. Instead of relying on CPU usage to determine what's happening, I would use the log. Change log level to "trace", and the search the log for "startPlaying".