my downloaded files say .m4a - what happened to .mp3?

You have to convert the files to MP3. You can do this in iTunes.

Preferences > General > Import Settings > Select MP3 & the quality you want.

Select the songs you want to convert > right-click > Create MP3 Version

kmhayen wrote:

I have a Dell PC running on Windows Vista. On this laptop, I have iTunes application for downloading music. I also have a Wondowsphone 8. I'm trying to copy my downloaded music from my PC to my phone, but am having trouble. I copied and pasted my music files in the file explorer windows, but the phone does not recognize the files at all. The file extensions on the music say .m4a, but one friend says that they should be .mp3.

Your friend is incorrect and you do not need to convert them to mp3.

-> http://www.windowsphone.com/en-us/how-to/wp8/music/syncing-music-faq

and -> http://www.windowsphone.com/en-US/how-to/wp8/music/copy-my-itunes-music-to-my-ph one