So I've got Sonar 8 in a Win 10 PC, and I can't hear what I play unless I increase the volume in my CASIO piano (It wasn't like this before) and whenever I want to insert a Software Synth, all of a sudden I can't record nor play anything, but as soon as I erase that audio track (that is used as the output of the software synth) I am able to record again, yet no sound comes from my speakers. I need to fix this as soon as possible. Thanks!

Midi meters metering?

Yes, they are

Sonar 8 and Windows 10,  hhmmmm!!
 
I wonder why he's having problems....

I've also tried Sonar X1 and I have the same issue

Can you play regular audio files from your project? If not, it sounds like a problem with your audio driver. Have a look at audio preferences and make sure you have the correct driver model and interface selected.

Yes, I can play audio files normally

Has anything changed recently?  Can you list the basic specs of your setup?

Are you input-monitoring the Casio through an audio track? If so, make sure Input Echo is enabled on the audio track. Also, regardless of how you're monitoring the Casio, either set the MIDI track Volume control to 127 to get max output from the Casio or right-click and choose 'Disable Control', and set the desired volume on the Casio.
 
If problem persists, we'll need a more detailed description of the track routing, I/O port and channel assignments and audio hardware setup. If it was all working, and you haven't rebooted lately, that might be a good place to start.

If you erase the audio track, you are removing the audio out portion.  Midi doesn't make a sound by itself.  You have to bounce down the midi track and the and audio file first. 
 
Insert soft synth
Create music
When done, leave audio and midi track in project and you will hear audio
If you want to remove the audio track, Bounce down the midi and audio together.
 
Done