Does platinum contain an mp3 encoder or do we still have to buy that separately?

see my answer in your other thread

It is a separate license. If you purchased an mp3 license for any other Cakewalk product, the internal mp3 encoder will be available. IOW, you only pay once for the license not each time you purchase a Cakewalk product. If you purchased a license and the current Cakewalk product does not automatically unlock the internal mp3 encoder, Cakewalk support will provide the necessary unlock software.

I just use Gold Wave which is free to try and cheap to buy. Wavosaur is free and will also do batch conversion. 
 
http://www.goldwave.ca/
 
http://www.wavosaur.com/
 
 

http://www.softpedia.com/...-Sonar-Mp3-Patch.shtml
 
Go to the above link for a seamless great way to output MP3 files. Install then run; it's an outstanding FREE method of exporting MP3s within Sonar.
 
 

Aaron's Sonar Mp3 Patch ...it works perfectly for me!
 

I just downloaded the "Aaron's Sonar Mp3 Patch" and it adds a loud hiss to the mp3's every few seconds. Anyone know what that's all about?

have you tried exporting the same file to wav, that's the first step?
 

The options are to encode to mp3 and discard wav or to keep the wav. I did keep it.

Just an update on this issue. Seems it wasn't the encoder causing the noise at all, but somehow my Amplitube plugin unregistered itself and was hissing every few seconds... Odd.