Go to Start, type dxdiag and open the application. Click the Sound tab, check your sound card model and download and install the latest drivers available from the developer web page. After that, go to Start > Control Panel > Hardware and Sound > Sound, right-click in the Playback tab and press Show disabled devices. Double-click the sound device and select Use this enable from Device usage option. After that, click OK, select again the device and press the Set default button next to Properties.
If it doesn't solve the problem, uninstall the application using its uninstaller or Add or Remove Programs feature (Control Panel > Uninstall a program) and run CCleaner or AML Free Registry Cleaner to make sure that there are no leftover rogue registry keys in your system. After that, download and install the latest SmsDiscount version. When you first launch the software, go to Tools > Options > Sound Devices and select your sound device from the Audio In and Audio Out drop-down list.