Google Music - Free song-matching to (potentially) upgrade and unify your collection - $25 from Apple and Amazon

Quote from ShengD : I think there's a misconception going on here. M4A is a container type for Apple's audio, but it is not a codec. M4P files can be either lossless or AAC (which is lossy). Lossless files are significantly larger than MP3 or AAC files, so if what you have is approximately the same size (ie 320 kbps or less), then it is certainly not lossless. Some people prefer AAC to MP3, but at such high bitrates, it is likely indistinguishable (despite audiophile claims to the contrary). Assuming your M4A and MP3 files are of a comparable size, they will be very similar quality. Perhaps there is a difference in the way they were encoded or the sources used, but the codecs themselves will produce nearly identical audio when over 256 kbps. No, MP3 only. It will try to match first, then if it cannot find a match, it will upload the original.

Thank you for taking the time to respond. I admit, I don't know much about this stuff, but I'm trying to learn. When I realized there was a noticeable difference in how an Amazon MP3 sounded and how an iTunes M4A sounded, I did a Google search and found the following:

http://www.m4a.com/m4a-vs-mp3/

They say "Apple devised a compression process that can still create much smaller audio files but without losing any of the original quality."

That's why I thought the iTunes M4A files were lossless. I guess it makes sense that they couldn't be full lossless files, those are very large. But regardless, there's a noticeable difference there. It's not so much "quality", like the MP3s sound "bad" compared to them, or are missing parts of the audio. It's just that the M4A files are louder and seemingly clearer, making it easier to understand lyrics. It's different than just turning up the volume, the words really stand out more in an M4A file. Try it out for yourself, download the same song from Amazon and iTunes, then listen to them both at the same volume level.

I'm even more confused now, lol. It seems like Amazon/Google will just switch most of my M4As to MP3s. I'm not sure what to do...