Many audio players can simply change playback speed, but the pitch is corrected to keep the music sounding natural. Like VLC or Windows Media Player and many others.
My intent is to create in real-time a sort of Vaporwave vibe (for slowed down tempo with lower pitch) or Nightcore songs (speed up-tempo with higher pitch).
That is the best analogy I can come up with. As you already know, many players can change playback speed, but they also correct the pitch right way.
I want the pitch to be affected together with the speed.
What I've tried, I used audacity to change the sample rate, not converting it, just re-sampling, like telling the playback engine that the sample rate of this audio is actually higher or lower that it is (depending on the desired effect) but that can't be automated to a batch process.