A few years ago, apple stopped encrypting music. The music is watermarked, but not encrypted anymore. These files (m4a), can be played without issue.

The older stuff, which is encrypted (m4p), can't be played. However, you can just remove the DRM using any of the available DRM removers, with no loss (they are just removing the encryption, not transcoding).

iTunes DRM tracks will likely not work, but at this point iTunes hasn't sold DRM tracks in a long time. Most (all?) other tracks have had the ability to upgrade and make DRM free for awhile now.

DRM will be an issue for any older purchased iTunes Music.

But you can pay a little more and Apple will provide non-DRM versions.