No problem. Just copied a few mp3 files to the SD card. (i've musicolet installed). I go to files, 3 stripes top left. Tap on the SD card and tap on the song.
I have been doing a lot of experiments using a combination of:
It does make sense, your conclusion. If it was Windows, it could be Windows own sound driver. The 'cheaper' apps use the systems driver and the 'better' apps use own drivers. Maybe this is also the case with Android.
Try AIMP, works great with FLAC format, other players were giving me a buzzy background noise