A short term solution I may have found (tested with Mic on a PC with Mic output disabled, so should work with the USB sound card), is to use
Audacity "playthough" (passthrough) option. While it records the input, it plays it back at the same time (delayed by a second or so due to software decoding, no biggie).
The only downside is the extra system resources it needs to be constantly recording, but at least you could save the recording for later (although TrunkView already does this).
Richard: Ok here is what I have tested to work. I'll start from the beginning in case you missed a step.
Under Sites Database, Add one call "Biscom" (or whatever you like), ID 0049, Control channel 415.925, base frequency 415.5625, step 12.5.
Under settings, have your sound card selected, and have mute selected on whatever input you used (this is where I've had the drama that it won't mute, so if it doesn't for you either you may be stuck with data noise as it won't be muted).
Under the RS232 make sure you have the correct COM port selected if you have the radio connected to this, and the correct baud rate (must match both the radio's own setting, and windows device manager setting). For mine I've also selected Uniden and ticked Dynamic Memory.
Now with all that set, and your scanner audio output plugged into your PC's input (you may need to select Sites > Biscom or whatever you called it to trigger your scanner to change to the control channel), it should be muted and have details on screen about the trunk, and when people start talking it should come up with Go To channels and ID's and such. By default your scanner should also change to the voice channel being used, and when the conversation is finished it will return to the control channel (I set the timeout in the options from 0 to something like 300, in case it gets stuck on a voice channel and doesn't return to the control channel by itself).
Now for your question. To filter out other ID's and only tell the program/scanner to track certain ID's do the following. Options > Profile editor. Create a New profile, and save as THE SAME NAME AS YOUR SITE (Biscom). That way this profile will load when you load the site.
Now you can alpha tag ID's. Prefix is always 002, ident is the caller/ID and label is whatever you put and add record. Now when you go to the main screen instead of ID's coming up your names will. As I said before add "002 6111 Stirling Group" and you'll see it clearly on the main screen.
Go to tracking, and under Include ranges, put prefix 002 from 1300 to 1399 & 002 from 6111 to 6111 AND TICK RANGES ACTIVE. I also put in exclude 002 from 5000 to 6000 to make sure the taxi's don't come up. Save the profile.
Be aware that even with the tracking ranges set, ALL ID's and commands will still come up on the home screen, however you'll notice it will only command the radio to change to a channel when an ID matches 6111 and 1300-1399 (our Stirling Security guys). If you want to listen to other ID's just go back and untick ranges active.
Any questions let me know!