^ That's the answer I was about to provide. You can't really compare the volume you get out of your D50 and the volume out or your computer. For example, I have four separate listening points and I get different volume and sound performance out of each...1) directly at the recording/recorder either during recording or directly from playing back the recording, 2) at the PC when I master, 3) on my ipod, and 4) on my stereo system. All four of these can sound different and have completely different volume levels, but I can confirm that when I listen though my laptop, it's the least impressive in terms of peak volume.
Bump your levels up from -12 so that they're closer peaking near 0 and you should be OK.
EDIT TO ADD: Don't be reluctant to adjust your levels in post upward from -12. Almost everyone does it (assuming you're running in 24bit). In the olden days of 16bit, people needed to run hot for best sound, so they'd ride the red line at 0db or -1db, but they'd risk going over for the sake of getting a better sound. Nowadays, almost everyone runs 24bit and leaves their peaks around -12db during the show so they can relax and not have to monitor levels every 5 minutes. The extra resolution of 24bit means that there's little or no 'hotness' lost by bumping levels up to zero in post. Yeah, you give up dynamic range, but the D50 is a low noise unit and if the music you record is fairly loud, your DR isn't much of an issue anyways.