Locally saved files won't play on smartphone

[Wobinsky]

Hello guys,

so i have the problem that i can't listen to any of my locally saved songs on my Sony Z3 Compact.

I synchronize the folder with the music tracks on my desktop computer and then add them to a playlist.

On my phone i make sure the Playlist is offline available. But whenever I play a song the time runs but no sound. When i try skip in the song the track jumps to 0:00.

All officially available songs on spotify work flawlessy.

Am i doing something wrong? The songs are .mp3 , nothing fancy

I already reinstalled the App and cleared cache.