1 Find and install audio/video conversion software. There are tons of programs available, many of them free via the Internet, but be careful. Some will only convert MP4s to MP3s and not MP3s to MP4s.

2 Determine which MP4 format your device requires for the file you're converting. For example, an iPod will see an MP4a file and determine it is an audio file. It will see MP4 files (with no letter) as audio and video, MP4b files as audio books, MP4v files as video and MP4p as protected files.

3 Download and install the MP4 codec. The audio conversion software should include this, but some of the free ones require this as a second step. See the link to downloading the proper codec in the Resources Section. You can't convert the files without the proper codec.

4 Select the file you want to convert. Some software will simplify the process by having you right click on the name of the file. You then select the format you want to convert it to. Others require additional steps.

5 Using more sophisticated audio/video conversion software, you may have to select the files by clicking on the MP3 files you want to convert, select the output format (MP4), select a bit rate or other options and then convert the file