Yes, USB 2.0 spec is 15 feet. You may get away with longer with slower devices. Also, you just need a cable that meets spec - $20 or so. The only way a USB can "change the sound" is if somehow bits are uniformly changed to alter the sound in a uniform way. Or some bits get flipped and some samples are corrupt and substituted in the player with previous good samples. It's quite easy to determine if your cable is corrupting data. Just connect an external USB drive to your computer and cable and copy files with some sort of data verification (checksums, zip file intergrity test etc) and see if any files get corrupted compared to the source when reading it back. It's very unlikely you'll see and difference between a $20 cable and a $200 "audiophile" USB cable.
