In Mixcraft, click on the File menu and select Preferences. On the Playback tab, select the correct sound card driver for the Playback Device option. Do not choose "Default Device," but the actual driver for your sound card.
If the problem persists, try uninstalling and then downloading and installing the latest version.
Also check to see if you've got the latest sound card drivers. Check your computer or sound card manufacturer's website for updates.