Jim, using the mouse to trigger the drums is just an audition feature - the audio is sent direct to your soundcard and bypasses the inputs of GT4 completely.
To check out the various ways of recording and inputting MIDI data into a track in GT4, check out this tutorial I put together in another thread:
The easy answer is no, you don't need a MIDI interface to hear sound from
Studio Instruments, neither do you need a MIDI interface or a hardware MIDI controller to input the data required to play them.
However, if you do at some stage wish to record MIDI information into your project 'live', you will need both a controller and an interface, although most controllers can connect to your PC and be routed into Guitar Tracks Pro4 with an appropriate USB lead.
The important thing for starters is to learn a little about what MIDI is, and more importantly, what it
isn't.
MIDI data contains no audio, but it does contain the information to 'instruct' a software synthesiser (often shortened to
softsynth) what to play - for example which notes should be played in a piano softsynth, when they are played, the force with which they are played (velocity) and their duration; similar information could be used to play other instruments.
Furthermore, MIDI data can be used to trigger the different parts of the kit in a drum softsynth.
Other than using an outboard controller, there are a few ways of getting the MIDI information into your project. The data can be entered using your mouse either in GT4's Piano Roll View (PRV) or using the Staff View, using musical notation.
The PRV opens in a window that contains the 'grid' into which the data can be entered and edited, as well as a keyboard which allows you to audition the sound each key is associated with - for example, if you're using a piano softsynth, using your mouse to press the middle C key in the PRV should play middle C in the softsynth.
Incidentally, some softsynths (such as Studio Instruments
Electric Piano and String Section) have their own keyboards which make it easier to audition the program and settings you want to use.
Others, such as SI
Drums and SI
Bass Guitar have graphical interfaces that let you audition the sounds - left click on a drum and you'll hear it play for example.
For softsynths that don't have such a keyboard or interface, you'll need to use the PRV.
Another way is to import MIDI data into your project - the most popular application for this is probably in drum softsynth programs, where pre-recorded drum samples are dragged directly into a project to form the basis of a drum track.
The beauty of all these methods (
including recording live with a controller) is that the MIDI information is completely editable at any stage - 'notes' can be individually altered for pitch/duration etc or deleted, or more notes can be added.
When you insert one of the Studio Instruments into a blank project, GT4 should 'wire it up' so the output audio from the softsynth goes to your soundcard, so you should be able to hear it when you trigger a sound.
Here's a couple of quick examples to get you going, open GT4 and select 'blank' for the project type.
SI Strings using the Piano Roll View From the menu bar, select
Insert > Soft Synths > Studio Instruments > SI String Section This screen will open, check the same boxes as shown:
The SI Strings GUI should now be open - left click on the keyboard at the bottom and you should hear some sound. Left click in the
Load Program field to audition other settings.
To input MIDI information in the PRV, first minimise or close the Synth Rack and the SI Strings interfaces (you can re-open a softsynth GUI anytime from
View > Synth Rack and clicking on the icon to the left of the synth's name). Now click on the PRV button:
Just to make certain everything's working OK Mitch, click on the
PRV Draw Tool and then use the left mouse button to click in the grid in a few places - you should hear the sound as the data is entered:
Finally, return to the
Track View - you should see the MIDI information you just entered; press the Play button and the music you just made should play back! Importantly, as MIDI is just data and the sound is coming from the SI Strings softsynth, try changing the project tempo around a bit - the notes will play faster or slower but they will
still play at the
correct pitch, i.e the pitch you entered in the PRV.
SI Drums using pre-recorded samples From the menu bar, select
Insert > Soft Synths > Studio Instruments > SI Drum Kit, check the same options as above and the Drum GUI will open.
You should be able to hear the drums by left clicking on them.
To audition SI Drum's different kits, click in the
PRG► field and select from the drop-down list. To audition the 'loops' and sequences first expand one of the five genres and then double-click (or press Play on the transport) on one of the patterns; it should play - to hear it over and over press the Loop On button on the transport. For example, choose the Full Throttle kit and then expand the Rock genre and select BEAT THIS Now for the tricky bit! To transfer this pattern into your project, hold down the left mouse button over the BEAT THIS name and drag to the left edge of the MIDI track (the bottom one of the three) of the SI Drum Kit track: 
It should now play back the same as in the Strings example above. To start putting together a drum track, you can use different sequences and patterns and drag them to wherever you want in the project Mitch - plus all this information is still completely editable in the PRV.
To quickly copy inserted patterns across your project (say you have a pattern you want to play through a whole verse) right click in the clip and select
Groove Clip Looping. Hover the mouse pointer over the right edge of the pattern clip until the pointer changes to an arrow icon; now hold down the left mouse button and drag the clip across your project - the pattern will be repeatedly copied until you stop.
Finally, you'll want to be able to treat your MIDI tracks with VST effects and processors in the same way you do with audio recordings, as well as being able to route the output through any busses and sends. To do this, you can Freeze the synth, this bounces the synth's output to audio which can be processed in exactly the same way as any other audio. The big benefit here, as opposed to recorded audio, is that you Unfreeze/Thaw the synth at any time and go back and edit the MIDI data again, as the freeze process doesn't delete the MIDI information! To freeze and unfreeze a softsynth, either click on the snowflake icon on the synth in the Track View or on the Synth Rack, or select the track and select Track > Freeze: 