If anyone can straighten this out for me... I'm flummoxed by midi in general, but I'm trying to learn to use the Akai MPK261 with Sonar, with Audiobox USB... I'm creating Soft Synth tracks, but they are only working intermittently, sometimes I get output, sometimes I don't even when I haven't changed anything at all... often, right in the middle of playing a part the sound goes away, though the part I'm playing still records and I hear it on playback. I've not been able to pin down what brings it back. When I create a track the I/O defaults to Output=Master, Input=Omni, then I can see the Channel, the Bank and the Patch I've chosen. I've played around with the I/O settings but this seems to be the only setting I get output at all with, and, again, it's intermittent (for instance right now, I get squat). I've been watching as many YT videos about Midi, but they tend to start at a higher level than where I'm at... any help would keep me from jumping out my window.

What softsynth and is it the mpk's keys pads or both?

Right now I'm just working with Cakewalk TTS-1, and I'm just playing it like a keyboard, can't even think about the pads at this point, I just need get sounds consistently (though I just banged on the pads and I get nothing there either, but sometimes I do).

The pads are likely set to midi channel 10.
I'm assuming you're just using the tts piano patch?

Gosh, I hope the specific patch doesn't matter that much... I've been trying Piano, Reed and Brass patches, percussion... again, the pads are for the future, I need to get working and sound is sort of useful and I'm not getting any, at least not consistently or reliably, so far this morning, nada.

The advice usually given to anyone with this issue is to set the midi playback buffers to 500 or 750 in edit-preferences-midi-playback and recording, I think default is 250

Yeah, mine's at 750, I'll try 500... to be clear then, the MIDI Output on Master, Input on Omni is correct?

Three things that pop to mind - 
1 you might have a keyboard split defined in the preset you're using
2 are you running a midi cable into the audiobox?  if yes, unplug it.  
3 have you plugged the mpk into multiple usb slots?
 
shut down your computer, turn off the mpk, start your computer and log in to windows then turn on the mpk and once it's fully powered up launch platinum
 

Lol, okay, did all that, now not only do I have no live sound, I have no playback sound of my already recorded tracks. I did have another Midi connected to me AudioBox, running to another keyboard (shouldn't I be able to do that?) so I unplugged that... can't have anything split on the keyboard because I can't get sound long enough to learn how to use it... and the MPK is only plugged in with its USB Midi.

Okay, restarted again, got playback sound back... but still no sound when playing the keyboard through the SoftSynth track.