I can hear jumps in the music when playing back a song

From N-Track Wiki

If, even with a few tracks you can often hear jumps in the music, try to increase the buffer size and number in the preferences dialog box. Increasing this parameter however will increase the time delay between any action done with the mixer and its audible result.

If the jumping seems to be independet from the buffer sizes, the problem may be related to the soundcard. Try to update the soundcard drivers downloading them from the manufacturer web site. If an update doesn't yet exist, try to reinstall the current drivers. Also, the video card may be interfering with the soundcard, so maybe also an updated driver for the video card may help. It may sound odd but because of this "interference" problems some soundcards record correctly only when using a low screen resolution with 256 colors. Try to disable the graphics hardware acceleration in the Control Panel/System/Performance/Graphics/ dialog box. The interference may also be with the hard disk controller. Try to disable or toggle the DMA channels on the Hard Disk Controller: go to Start, Settings, Control Panel, System,and Device Manager. Double click on the CD-ROM drive, then go to Settings, uncheck the DMA option. Follow the same procedure, to disable the DMA options under Disk Drives, and under the Hard Disk Controllers. If the computer BIOS allows it, try to increase the "PCI Latency timer" setting. Try the Windows 9x Setup tips.