For Mp4, I use an app called Hi Jack It!, and audiohijack which re-encodes (unfortunately in realtime) to mp3. I don't know of a current app to just break the PlayFair DRM. There were some older ones released by Jon Johansen, that would strip the DRM and make them normal m4a files. It's been quite sometime since I've looked though so maybe there have been updated for the newer versions of the DRM.
They're both mac only but, http://www.rogueamoeba.com/audiohijackpro/ and http://web.mac.com/spkane/iWeb/Actionable%20Intelligence%20Software/Hijack%20iT!.html I imagine there are PC equivalents.
If they have drm, you have to use something that "re-records" them in real time (I don't know if there are free tools for that though.) or burn them to an audio disc and rip it. If you bought the files more recently, the files are drm-free and most modern converters should handle them.
When dealing with uncommon (to me) formats, I usually use mplayer ("mplayer -ao pcm:file=output.wav input.m4a") to get a wav, then stick that into oggenc/lame/whatever. This way I can convert anything that mplayer understands, which is quite a lot.
Transcoding one lossy format(m4a) to another(mp3) results in awful sounding music with significant fidelity loss. Just rerip your CDs.
Storing audio at 44.1 kHz severely limits fidelity. Buy everything on vinyl instead. Seriously though, very few people have anything close to the "golden ear", so while it's technically true there's a loss of quality, most people don't care - and, with the right settings/bitrates, most really can't tell the difference. (BTW, while you're transcoding anyways, I'd recommend using OGG Vorbis instead of MP3. Way better at the same bitrate, or a lot smaller at the same quality, whichever you prefer. Only drawback is that there's not as much portable players supporting it as there are for MP3s.)
Back when I used iTunes, I used a program called JHymn to take care of the conversion. http://www.softpedia.com/progDownload/JHymn-Download-48754.html
There is a way that I used to do it, but it ends up disabling your iTunes account. I think there are players out there that do play .m4p files from iTunes.
Well unless all of your music was purchased pre-iTunes plus the the should already be DRM free as Apple stopped adding the fair play DRM crap. The should be standard AAC/MP4 files that a good chunk of devices and players support. If they are fair play protected DVD John had a working fair play stripper you could try.
