Is there an App for iOS that will change the key of audio tracks ?

My aunt loves to sing-a-long to YouTube music tracks... but sometimes the songs that she likes are in the wrong key for her voice... is that an app out there that will work with YouTube to an independent audio file to allow her to change the key.

Or is there a better way to allow her find a song she likes on YouTube and change that song to a more comfortable key for her voice.

If she was to try and extract the audio from a YouTube video, what again would the best and least expensive app solution out there. She's in her late 60's, so app UI simplicity is essential also.

Any suggestions or comments most welcome.

She has an iPhone and iPad so iOS is bets platform for her.