Distortion when rendering from an MP3

I have imported an mix in mp3 format. My project setting are 44.1 at 24 bit. It does this automatically when I start a new project and imported the media. I have the box checked that allows media to change the sample current sample rate if necessary. It sound great when play it within reaper. However, when I render it to a .wav, it sound shitty and all distorted. I'm doing a compilation christmas CD so I need everything to ultimately be in .wav format for mastering. Any suggestions please!

Mp3 is very destructive lossy compression. Converting it back to a wav file gets artifact-y. You really can't use a low quality portable copy of a mix/master for production like this. (I mean, you can... and sound comes out... but the fidelity has been decimated.) Tell the client to deliver the actual master to you. At least the 16/44.1 CD reduction (which is already portable quality). Better would be the full quality 24 bit master. Or the raw mix (24 bit, original sample rate) which would allow you to master the entire compilation.

Re; mp3 mastering 

Thanks for the advice, I emailed the composer of the .mp3 in question and requested the master, or atleast .wave file. So the bottom line there is no way of resurrecting an .mp3, even if you convert it back to a wave file. Is this correct?

The guy with the funky MP3 

Predator [is that you real name? ;-)], rather than using Reaper to convert, try one of the various conversion tools available on the Internet. Try VLC's Convert option on the file menu for starters. Alternatively, I have found that the following works well: play the mp3 file, and re-record the audio to .wav. Sounds like a crazy idea at first, but works well, probably due to some of the artifacts created by lossy compression being removed in the process. El-Rallef