The Transit will bypass your existing soundcard, depending on your operating system settings. Actually, under your "Sounds and Audio Devices" in the Control Panel, there is a tab labeled "Audio". It has drop down lists for your "Sound Playback" (soundcard output), "Sound Recording" (soundcard input), and "Midi Music Playback". Alternately, this same settings window may be accessed by selecting "Audio Properties" from the speaker icon in your Windows desktop taskbar, if you have it showing (Windows volume control).
When I first read the glowing reviews about this device, they were some that implied you could not have more than one soundcard loaded on in a PC. Not so - it simply becomes another choice in the drop down lists for possible sources/outputs.
Yes, the volume is controlled through the Windows operating system.
I researched it for a long time, and tried a lot of the standard alternatives - including several types of Turtle Beach/Creative/EMU internal soundcards. None of them had the super low-noise and transient dynamics that exist in the Transit. It is far superior to a Turtle Beach Music Advantage or Roadie, and it puts all but the most professional internal soundcards to shame. For instance, there are posts in various forums that have tested it better than the $200 Creative Platinum with the separate inputs/outputs card-face.
The drawback, as stated, is that it is only 2-channel, unadulterated 20-20K, 100+db SN stereo - no dolby, special effects or speaker capability. However, as a pure source for your expensive or not-so-expensive head amp and headphones, it can't be beat for the price.
Think: audiophile-quality CD/mp3 player from your PC, and that's it.