Oftentimes, when I'm playing the opening measures of a project -- synths only: SoundCenter, TTS-1, etc. (no Audio) -- the playback is fuzzy and there's static, and sometimes slows down a bit. It cleans up after a while, but it's always in the beginning of the song. Does anyone know why this happens, and how do I fix it?
 
I don't yet have an audio interface, but my otherwise, my PC is "up to code".
 
Any insight is appreciated! Thank you in advance.

Try adding a few empty measures to the beginning.
By the way, if you're using soft synths, you're using audio.

I find this happens if the sample rate is set differently in Windows sound devices and Sonar (e.g. 48Khz in windows, 44.1 in Sonar).
 
Stopping/Starting the audio engine in Sonar after you've started up will normally fix this - or just go into Windows Sound Devices and make sure the sample rate is the same as it is in Sonar.
 
Windows does have a nasty habit of changing this after updates.
 
M.

This is a good reason for rendering midi to audio at some point in your process when you are comfortable with the midi part. Can always give the rendered midi to audio song a new name, as to preserve the original song with midi(vsti's).

I disagree the the above post. There's no reason to convert midi to audio.  Midi takes up very little hard drive space and myself I've never had a reason to freeze synths. Those days where when we had under powered computers and long gone unless your scoring a symphony :) 
 
Have you tried using different audio modes . WASAPI exclusive works the best of the supplied drivers.
I use Asio4all on my laptop I use without a proper interface for pure midi work. I found it performs better than WASAPI. It's lke having a proper ASIO set up even though it's really using WDM mode as a wrapper.   If you plan on recording any audio you will need a proper interface for sure to avoid timing issues. 
 
Greg what the OP is saying is there are no Audio tracks. 

Grundberg
Does anyone know why this happens, and how do I fix it?
I don't yet have an audio interface, but my otherwise, my PC is "up to code".

 
Get an audio Interface with ASIO drivers. Buffers will make a world of difference. 
 

Cactus Music
I disagree the the above post. There's no reason to convert midi to audio.  Midi takes up very little hard drive space and myself I've never had a reason to freeze synths. Those days where when we had under powered computers and long gone unless your scoring a symphony :) 
 
Have you tried using different audio modes . WASAPI exclusive works the best of the supplied drivers.
I use Asio4all on my laptop I use without a proper interface for pure midi work. I found it performs better than WASAPI. It's lke having a proper ASIO set up even though it's really using WDM mode as a wrapper.   If you plan on recording any audio you will need a proper interface for sure to avoid timing issues. 
 
Greg what the OP is saying is there are no Audio tracks. 

It's just a matter of opinion and workflow.
Different workflows and procedures for different people. I was just throwing an option into the pot, not an end all be all. Work flows are subjective, but I understand your strong opinion.
That being said, if "there's no reason to convert midi to audio", why does this function exist?
 
 

"That being said, if "there's no reason to convert midi to audio", why does this function exist?"
 
Probably a holdover from the days of limited MIDI tracks (?) and lack of multi-timbral synths, and before synth freezing was introduced. And slower computers with limited RAM.
For what it's worth, I always bounce my soft synths or record my keyboard before final mixdown, even though it's not necessary nowadays.

Converting midi to audio is critical for back up purposes.
I always back up my midi tracks to audio. In 2030 when VST6 doesn't support VST 2, I wont need to load up a 20 year old version of Sonar.

Sorry if it came across sounding the way it did I could have phrased it better.
The point I was trying to make is you do not need to freeze synths anymore as far as Sonar ( any DAW)  and a computers is concerned. Now a days or DAW can handle a LOT of VST's playing live.   Sure you can freeze them for reasons like Chuck's but I guess that was not what I was getting at. 
 
 I'm the opposite as I want the midi in the future because the sound of Synths just keeps getting better in my world. 
I still use midi files I made back in the 80's and they are certainly future proof. The sounds I used back then are still available to me but I would never use them. My first system was a Korg Poly 800 for bass and a Roland 505 for drums. I still have the 505 and I just found a freebie Poly 800 VST. 
 
I think a big part of this is that for each of us is what types of sounds we are using too. Traditional vs weird synth sounds. 
Traditional being drums, Bass, Piano, organ, flutes,  trombones etc. Those sounds just keep improving over time so as long as we have the midi our old projects can be called up and improved using newer VST's. . 
But if your into weird synth sounds then defiantly you'll have trouble re-creating that using a different VST down the road so an audio back up is a good idea.