Hi, I downloaded a few midi's from Hit Trax, and in a few of them when I insert the SI Bass synth and send the existing bass track to it, it won't play back. I noticed it seems to be on tracks that contain things like wheel bends and such. Deleting everything except the actual notes doesn't seem to help. Any clues?
 
Also, are we able to attach files here? I can't seem to find a way to do so.
 
Thanks!

take small steps and problem solve it.
1- Do you see any meter activity on that midi track ?
2- If you draw in notes manually in the piano roll view, can you hear sound ?
3- Is it just those midi files from hit trax your having issues with ?

Also, have you confirmed that the notes on the hit trax base MIDI track are actually within the range of SI Bass synth?

Do you hear the Bass Synth when you play on a (virtual) keyboard?
Are the notes on the right MIDI-channel? Maybe the clips send on channel 4 and the synth receices only channel 1?
What happens when you use an other synth plugin?
 
 

MIDI channel would be the first thing to check. Although most MIDI files will default to channel 1, a different channel number may be specified within the file. SONAR lets you force the MIDI channel, overriding whatever's in the file. Be sure to actually specify the MIDI channel in the track header rather than letting it default to "None". Most synthesizers default to channel 1, but there's no law that says that has to be, so also check the synth's settings. 
 
Use the the File->Import->MIDI dialog and browse for files rather than dragging them into your project, because down at the bottom of the dialog it'll tell you whether it's a multi-track file. When importing a multi-track file, the MIDI channel(s) will be specified in the file. The bass part might be one of two or more tracks all bundled into one file. So make sure you're actually getting the bass track and not a bass drum track.
 

Assuming the notes are within the playable range for SI-Bass, often times MIDI files not created in SONAR contain data that shut down the sfz engine. Here are a couple of ways to deal with this issue  http://forum.cakewalk.com/FindPost/3235786
 

Another way to trouble shoot that comes to mind is
First go to Preferences/ MIDI devices and unselect all outputs for midi.
This will then automatically insert the TTS_1 when you OPEN any downloaded midi file.
So it will play without issue as a multi track, multi channel song. Never had this not work for me.
 
Now track by track you replace the TTS_1 with other instruments.
I use the SI bass a lot and always you have to transpose up 12 or it will be below it's range for many notes.
And it runs on what ever the channel originally was as far as my experience goes, I've never had to change the channel.
What scook has mentioned is then the only other reason it goes silent.
That's easy to test by just using the GUI to test it for playback, pluck a note.