Again can you say where they are? As far as I know if you put your own mp3s into a playlist on your pc and then sync that playlist to your phone then those tracks on your phone are in spotify format ... on your phone you cannot play those tracks in any other app (unless you transfer them again in mp3 format in which case you cannot play those mp3 format files in the android spotify app).
You are right that the files on the pc arenot converted and you can still play then on your pc in any player but I don't think thats the question being asked.
If you know differently then please say where those files are.
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