Your soundcard (also known as an "audio interface") is an important part of the process in making music with virtual instruments. When working with VI's such as Virtual Drumline, you are requiring a lot from your computer - much more processing than you normally require with other everyday tasks like email or listening to MP3's. As such, it's important that your audio interface is geared to handle these demands. "Stock" consumer soundcards that come pre-installed on many PC's often don't perform nearly as well as something that is designed to actually handle pro audio.
A little about latency:
Latency (or "audio buffer" size) is a term you will encounter often in audio software. Essentially, latency refers to the time it takes from the moment you (your software) request a sound to the moment you'll hear it coming out of your speakers. Your musician's intuition might tell you to keep this latency value at a very low reading. If performing live, this would be true. However, if you're sequencing in step-time (not in real time), it's generally a better idea to set latency a bit higher. When latency is lower, it places a much higher burden on your CPU and audio card. If you are trying to push too much audio via a low latency, it can result in pops, clicks, and other undesired results. If this happens, it could mean your latency is set too low or you're possibly using an audio driver format that isn't able to process all the data you're churning through it.
Windows (PC) users:
There are generally three types of audio drivers you may find with your soundcard.
ASIO (Audio Streaming Input/Output) is an audio driver architecture developed by Steinberg. ASIO drivers offer low latency and support multichannel audio cards. With its high performance and low latency, the ASIO format is highly recommended.
Direct Sound (DS)
DirectSound is a format designed by Microsoft and is a component of DirectX 5.0 or higher for Windows 98 or higher. Whether or not DirectX works well depends on the soundcard you are using. This is a common driver setup for Windows but does not usually offer the best performance with latency, particularly if the audio buffer size (aka "latency") is set too low.
MME is the standard "Wave" driver in Windows. Most soundcards support this interface and work with it quite well. However, MME is even less suitable than Direct Sound for real-time applications since its latency is comparatively higher. Since Direct Sound is so common, using MME isn't advised unless absolutely necessary.
Virtual Drumline is compatible with all of the above audio driver formats. As you'll see, however, ASIO is definitely the recommended driver choice. Using a soundcard that supports ASIO would be advised if you plan on playing dense scores in real time.
All Macintosh OSX systems ship with the standard CoreAudio driver format which generally performs quite well. If you later decide to upgrade your soundcard, most come with included CoreAudio drivers which make it compatible directly with OSX.