Playing a large mp3 file (e.g. audio book) it would be good if the last played location was retained from session to session and the ability to advance/slide to another location in the file was added (like changing the volume).
I see that this lack of slide is common to all files being played from my NAS or local PC (windows media player and iTunes) but not from Amazon Music. Why?
The SoundTouch product manager did an AMA with the community a few years ago and I asked him about the ability to 'slide'. He seemed to imply it would be coming but years later it never has, and now that SoundTouch has been discontinued, I highly doubt it ever will. I think it does work with Amazon and Spotify because Bose uses their API's, whereas they'd have to do implement sliding for MP3's on their own. The only way to slide with your own MP3's is to play the MP3 to the SoundTouch speaker via BlueTooth or Airplay 2, not through the SoundTouch app.
That said, I believe if you turn off the unit while its playing your own MP3, and you turn it back on, it will resume where it left off.