Download transposed MP3

Hello everybody! :),

I want to download the mp3 of for example Schubert - Schöne Müllerin for a sing along.

With the App i can play it and have no problem to transpose it two half notes lower so it fits to my voice.

But when i download it the mp3 is still in the original key nevertheless I transposed it before in the app.

Do you have any advice how it could work or a hint where I can find a playalong instead?

Kindly regards

Felix