How to replay other tracks while recording through microphone and hearing both.

Normally you'd monitor the mic feed directly from your sound device, whilst singing to the replay. It's possible to get the track you are recording into the playback feed by using the I button next to the track Record button, but there will be a latency delay due to the fact that the signals have to be digitised, passed through the system and then got back out again - it's called latency. And if you have this delayed feed in your headphones it's virtually impossible to sing to. So the norm for monitoring yourself is to use direct monitoring.

There is a neat trick you can do to improve things a bit, though. You can set the mic channel up so that there's a bit of reverb on it, and then send just the wet effect out to the monitoring. That way you get your direct sound from the sound device, but slightly delayed reverb back from what you are recording as well. The delay in that doesn't matter, because reverberant sound is generally slightly delayed anyway. The track, though, is being recorded correctly and dry, as it should be - it's just the monitoring it affects.