At the risk of sounding like an evangelist, Rockbox has “timestretch” features, so that you can vary speed and pitch independently of each other, and in small (percentage) increments.
So you can speed up podcasts and audiobooks without changing the pitch. It doesn’t work too well with music, but it’s fine for voice recordings.
I can’t say what extra demands this places on the CPU, or how it affects battery life, but it just goes to show that it’s achievable and it’s been done already.
