could be several issues.
I'm no genius when it comes to digital recording, but . . . .
check your sample rate and depth. A lot of people are recording at 24bit (though 16bit is considered CD quality.) So if you're interface or software is set to record at 12 bit or lower, you may well get that "nintendo" sound. Also check your bitrate. The more times per second you sample, the better sound quality you will get. So if you are taking 44,100 samples per second, you are said to be recording at 44.1k. So . . . I don't recall the different common settings here. But check to see that your computer/interface are getting the best "picture" possible, and that you are taking as many of these "pictures" per second that you can. I hope that help.