Welcome to the forum… 
You are going to have to provide a few more details. Like… Do the individual tracks sound clear? What, if any, VSTs are you using? Are they activated? How are the VST connections set up? Anything of more detail will assist us to assist you.
If the individual tracks sound OK , then it is most likely that you have some VST (like a reverb or compressor) assigned and activated on the main stereo out channel… or… I don’t have the UR12 AI but, it is possible that some internal effects are activated in the UR (other interfaces have this, not sure about the UR though). These might be heard when playing back too.
Regards 