hi.
 
some of my midi sounds suck (especially the "ac piano"). how can i get better midi sounds? i only use the "standard" sounds on my sonar 8.0. i mainly do pop and singer/songwriter stuff.

You have never tried the 1,000 of excellent VST sounds that are included with Sonar? 
And you should upgrade to Cakewalk by Bandlab so your up to date. No need to run old cracked version no, it all free! 
The TTS-1 is a GM synth that is very lo fi and a good way to pre view a midi file. Nobody would use it much past that point unless they enjoy lo fi :)  
From there you replace each track with a better VST. simple. 
 
 

As the monkey said, for sure there are much better sounds than TTS-1.
In my signature is a link to a tutorial that explains how to start with a TTS_1 song and upgrade it. 

Use better sounding VSTs.

bdickens
Use better sounding VSTs.

+1
 
What's the point of having VST support in a DAW if you have restricted yourself to TTS-1 (the built-in GM soundset)?  There are hundreds of VST instruments out there, ranging from free to hundreds of dollars. 
 
If you don't want to spend any money in the beginning, that's not a problem.  Do a Google search for "free VSTs"... some of those instruments have proprietary format libraries like sfz, sf2, UVI, Kontakt, all of which have extensive libraries out there -- again, many at no cost.
 
You only need to dip your toe into the VST pool...