Gotcha. Well there are few problems going on here...
When recording audio, especially going from analog to digital, it's all about delivering a strong signal, clean signal that doesn't clip (clipping = greater than 0dB in digital audio).
It's been a while since I've used audacity, but I believe it does have an input recording meter/monitor that shows you how "hot" or strong/loud your signal is. I would try to peak around -6db so you have a bit of headroom for processing later.
Please don't take offense when I make the following statement: the sound card you're using isn't good...at all.
Try to visualize the audio signal path: For starters, you're using a two to 1 (RCA to 1/8") cable. RCA's are unbalanced (not-grounded), so you're signal has already begun to deteriorate. From the RCA's, it travels to the little 1/8" where your audio again has been deteriorated as the true L/R information has been squished into a single source (1/8" male). That signal will now be fed through your audio interface (Sound Blaster) and will become 1's and 0's that make up your recording as well as the audible playback coming from your speakers.
Part of delivering a quality signal to a recording device is making sure you have a quality means to get the signal from point A (DJ mixer, hardware synth, etc.) to point B (your audio interface/computer). Balanced TRS cables, XLR cables or in your case because you're using a Pioneer 400,
RCA to 1/4" cables. While they are not balanced, you will still get a better signal this way then using 2 to 1.
The other part of audio recording that is essential in terms of obtaining good quality is choosing the right interface. More expensive interfaces typically have more inputs/outputs in addition to hi quality preamps/A-D D-A converters. That said, there's nothing wrong with getting a "cheaper" audio interface so long as it meets your needs. This
Presonus box for example is only $150 and gives you the option of using either 1/4" or XLR cables and the box itself has some rather quality preamps/converters.
Yes, you'll have to spend a bit of money, but the end result/product will be worth it. If price is an issue, I find that bargain hunting on eBay can turn up some excellent deals.
Cheers.