Cant get sound to play through computer.

If the TASCAM is using ASIO drivers, change the driver mode to WDM or MME then select the computer's sound card as the Playback Timing Master under Preferences and the sound card as your output in the tracks and/or Master bus.
I'm betting it won't sound the same.

I have a couple of audio interfaces, and at the moment my desktop is using an old Native-Instruments Kore interface - quite ancient, but works.
 
On that interface, if I want to listen to output through the headphones plugged into the interface, for the project I have loaded I have to change the Output assignment for Master from the main outputs to the outputs of the headphones.
 
Could it be something like that?  Load up your project, then click on the Output assignment arrow on the Master bus, and see if it has another option that points to Headphones for output.
 
Just a thought - and it is how this particular interface works, but not how my other interface works.
 
Bob Bone
 

Well, I choose to run with the Windows Default Audio Device set to be my HDMI output of one of my 2 HDTV's, which have their own speakers.  That keeps all non-Sonar sounds completely away from my audio interface.
 
For Sonar, then, and other stand-alone music programs - Dimension Pro, Rapture, etc., I have them set to point to the ASIO drivers of my audio interface, but then I also run only one of those kinds of programs at a time.  (the stand-alone versions of synths, like Dim Pro, etc., are the versions that run by themselves outside of Sonar - NOT the plugins for these programs, which get inserted into Sonar - just clarifying)
 
Anyways, so Windows sounds - HDMI, Sonar - audio interface.
 
Hope that helps, 
 
Bob Bone