Hi,
Here is what I would suggest this point - follow Ricks suggestion about sampling rates, turning up the audio level, adjusting your scanner position for best receive, etc.
Keep in mind that with your scanner hooked up to the computer and nearby the computer, you could be getting some noise induced on the line that is causing problems with a decode.
Manually use Tune to scan between 851 and 868.975 - find some active control channel frequencies for systems you can identify (by ear) as Motorola or EDACS. Find strong ones, that give you a completely strong signal. Do this while your scanner is somewhat near your computer but not plugged into the computer. Note the quality of the audio. then once you hook it back up to the computer listen to hte audio (not through the discrim/sound card but through the scanner's speaker) and see if you notice any change in the quality (decreased sig strength, noise induced) - then monitor the discriminator output for same.
Any Motorola or EDACS channel that you know is Moto or EDACS is going to be decoded by UT, whether the system is imported into UT via RR or whether you don't import anything at all. It'll just pop up as a new system and will show info.
So just do that with any frequencies you can find that are control channels of systems UT is known to be able to decode.
I know it sounds like a lot to do, but it's a logical next step. When I installed by tap in my 996 I initially did something wrong that ended up causing noise to be induced into the audio as it was fed into the computer and it caused problems - If I touched the ground on the 1/8th jack I could decode and if I let go I got all kinds of crappy noise induced. I forgot what I had done wrong at that time.
If you have SoundMAX audio, I think you have onboard audio on your computer (and perhaps an Intel board). At any rate, ricks mention about sample rates and such is because not all sound cards are created equally, and onboard sound cards often [at least to me] seem to be of lesser quality. Although I do know my on board card on my Intel motherboard (which also uses SoundMAX drivers) does decode fine.
Like I said, really forget about importing anything into UT at this time - instead focus on getting a strong signal with no strange noises on the signal - get used to hearing a "proper" Moto or EDACS control channel. Try tuning to many of them and see if UT will respond to any of them while "Running". I think I heard some people say that they had seen problems when switching control channels while it was running - I never have a problem, but you may want to Stop the receiver, switch to another active CC, then start the receiver again.
If it is decoding at all, on a control channel for a system that you do not have any imported data for, you'll start seeing new windows pop up as it starts to identify characteristics of the system. That'd be a telltale sign that it is working, at least somewhat.
Next step may be trying to record some of the audio (using your computer, maybe the Sound Recorder under Accessories - I thin that is where it is). Record a short sample sound bite, save it, then if you have someplace you can upload it to you should do that and then provide us the link so we can hear it.
Mike