Often, we have demo tracks or backing tracks that are not in the correct key. Now there is an excellent online resource to transpose your audio files and chord charts easily.
Transposr.com allows you to copy and paste a text chord chart to transpose to any key in one of their apps. Another app at the site allows you to upload any mp3 which will be transposed to the key you specify and generate a downloadable file for you.
The Audio Transposer
The Chord Chart Transposer
Is there an app you use for transposing mp3s and chord charts?