Sound from computer speakers

What's the TASCAM device?
That could essentially be your sound card when recording.
Does it have outputs for a headphone and/or speakers?
Try plugging into those and you will, hopefully, hear something.

You are correct in that SHS6 is a very capable program. Even though I have Sonar 8.3 and X1, I work in SHS6 daily.
After reading your post several times, I believe your problem boils down to some basic routing and setup issues.
This has several layers, so I am going to break your post down and try to answer each question separately.

I have had Sonar Home Studio 6 for some time and have just had time to try to figure it out. 
Initial setup is possibly the most brutal part of the learning curve. Once you get past this part, it gets easier. Well, sort of.

1.I have a Tascam interface hooked up by usb to the computer 
Ok, this is your sound card. That's very important. I am guessing you are using the ASIO drivers for the Tascam, since the Sigma Tel is not available as an option. That's a good thing. You want to use the Tascam exclusively.
Click Options>Audio.
Under the General tab, make sure the Tascam is selected as both record and Playback Timing Master.
Set your Sampling Rate to 44100.
The Buffer Size slider will be locked if you are using ASIO, but it should show about 10 msec.
Now click the Drivers tab.
Put a checkmark in all I/O boxes (they should all reflect the Tascam).
Click OK.

2.and then connected to a Yamaha keyboard for MIDI.
The Yamaha is connected to the MIDI I/O ports on the Tascam, correct? That should be fine, although Yamahas are notoriously finicky about MIDI connections via USB, and this may be playing a part in the key range thing.

3.Inputs and outputs are connected normally.
Maybe, maybe not.
Do you have the audio outputs of the Yamaha connected to the Line Ins of the Tascam? It's all about signal flow.

4.I am hearing the selected sound through the speakers of the keyboard but cannot hear the loop track or the metronome click.
Now we are starting to get to the meat.
You have the MIDI output set to the Yamaha via the Tascam MIDI Out port, correct?
This means the sound is being created in the Yamaha. The MIDI just tells it what to play.
To hear the sounds from the Yamaha, you need to send the audio output to an audio track in SHS6. See #3.
-OR-
And this where it gets really fun, you can use a softsynth.
This is the crux of the biscuit. The reason you hear no sound from the speakers, is there is no sound to hear.
If I am reading your post correctly, your sound is being created outside of SHS6, and you have not provided a path for the sound to get back to SHS6, and to the speakers.
Are you with me so far?
Again, signal flow. It's all about signal flow.
This is the part that trips up most new users, but once you wrap your head around it, you can find your way.

What is the loop track? MIDI or audio?

5.cannot hear the metronome click.
Use the audio metronome, not the MIDI. This should output to your interface.


6.I cannot get any sound from my computer speakers. 
If you are set up properly, you won't.
Remember, the Tascam(or any recording interface) is exclusive, using ASIO.

Try using a softsynth (TTS-1) Do tutorials #1 and #4 (I think).
Among other things, once you know how to work with soft synths, it opens up a huge world of sound.
Sorry for being so long winded, but I hope that helps you get pointed in the right direction.