Feature description:
Do you remember Spotify Running? So basically that.
Use phone to determine the running pace, and match music that is played to it.
I think you have access to tracks BPMs, you already have possibility of modifying tracks speed and pitch, so if the running pace is slightly different than currently played song (within 1-5%) just automatically change the speed of the song, but if the difference is substantial, search for a song with BPM closer to desired and do a soft transition.
Problem solved:
Just a feature request, Spotify killed this amazing functionality. I have been looking it for years since then, and haven’t found. That is why I decided to ask for it.
Brought benefits:
I think people that like to run and listen to music, will appreciate that, and when the word spreads out, this can bring more people to use Symfonium.
Other application solutions:
Currently, I don’t think this exists anywhere, maybe RockMyRun had this, but their music is… unique 
Additional description and context:
Screenshots / Mockup:
BPM is tag based so requires users data that 99,99% of people will never have. And that’s just one little part of the issue.