If you are looking for royalty-free tracks for a project, several reputable sites offer high-quality "slow motion" style audio (ambient, lo-fi, or cinematic) that can be downloaded as MP3s:

: A free web app that allows you to stretch time (slow down) or shift pitch in real-time and save the result as a new MP3.

If you have an existing MP3 and want to slow it down (change the tempo) without altering the pitch, you can use these tools: