How to convert the files from AAC to Mp3?

AAC is a compressed music format, meaning bits of audio data have been sacrificed in order to get a CD audio file into a manageable size for an MP3 player. To most people, the results of this compression aren't even audible. But converting from "lossy" AAC to MP3 (another lossy format) means a little more data vanishes into the ether. It's only small, but it's worth bearing in mind.
We advise that you encode in the highest possible MP3 bitrate (320Kbps) in order to minimise this loss of audio fidelity.
Ensure you download the iTunes Plus version of songs from the iTunes Store. These are DRM-free and can be easily identified by a little plus symbol next to the "Buy Song" button. Consult this story for detailed instructions on how to ensure you're getting iTunes Plus versions of songs.
1. Setting

Make sure you've got the latest version of iTunes. We're using version 8.0.2 on Windows and OS X.
By default, iTunes likes to convert your music into AAC, not MP3, so we need to change that. In the iTunes menu, click "Edit > Preferences" (or "iTunes > Preferences..." on OS X) and select the General tab.
Next to where it says "When you insert a CD", click the Import Settings button. Then, with the "Import Using" pull-down menu, select MP3 Encoder. In the pull-down menu underneath, select Custom. Select "320 kbps" in the "Stereo Bit Rate" menu, check the box to use variable bitrate encoding, and select Highest from the Quality menu. Leave everything else as it is.
Click OK on all of the open windows and return to the main iTunes interface.

2. conversion


You can convert all your files in one go if you like, but for the purpose of this tutorial we're going to convert a single file. The process for converting multiple files is identical though — just highlight all the songs you want to convert.
So, with your song or songs highlighted in the iTunes library, right-click on any one of them and choose Create MP3 Version. iTunes immediately begins converting your songs from AAC to MP3 using the settings we just altered.
The conversion process could take anything between a few seconds to a few hours depending on the number of songs you're converting and the speed of your computer. When complete, you'll have an AAC and MP3 version sitting side by side in your library. If you don't want to keep the AAC original, you can delete it now.

3. locate your files

Your new MP3s sit in the same folder as the original AACs. If you know where this is, all you need to do is locate them and copy the MP3s into any folder on your computer, drag them into your favourite music software, or on to your MP3 player, and you're done.
If you don't know where the files are located, simply click Edit from the iTunes menu, then select Preferences (or, on OS X, go to "iTunes > Preferences"). Choose the Advanced tab, and look at the "iTunes Music folder location" box. This is the folder on your hard drive that iTunes saves your downloads to. Navigate to it through My Computer or Finder, and remember it for future use.

And that's it! Your iTunes Plus downloads are now in MP3 format, and can be played on any device that supports MP3s.