Just using Google Play. If I have my SD Card in (which has music on it) I get a notice saying error: can't play sideloaded music. If I take the SD Card out it plays the music just fine.

It seems like a few people responding to the OP are trying to be helpful but are confused about what you are saying is happening. But I am experiencing the exact same thing you are. If I try to play a song from my GS4 that has that song on the SD Card, I get that same error message about not being able to play sideloaded music. If I try to play the exact same song from my Nexus 7 where it is not stored locally, it works perfectly. The song is in my Play Music cloud which is why I can play it from the N7. But since it is also stored locally on my GS4, I get the error message.For me, this is simply an annoyance right now. I can play songs from my N7 or N4 with no issues because I do not have songs stored locally on those devices. I just cannot play those songs from my GS4, although songs not stored locally play just fine even from my GS4. However, for somebody with only one device who stores songs locally, I can see how this would be incredibly frustrating. Hopefully this is something Google will fix in an update. My fear is that Google might never fix it in an effort to push everyone to the cloud only. I GET IT BIG G! The cloud is great. I live in it. Gmail, Drive, Voice, Calendar, Play Movies, Play Music, etc, etc. I use and love it all. But I also like the option of playing my music with no internet access. But this should not hinder me from playing music I have in the cloud from a device that also stores it locally.