In order to fix the problems, open the Start menu, type dxdiag, and launch the utility. Click the Sound tab, check your audio card model, and download the latest drivers available from the developer's page. After that, press Windows + R key combination, type devmgmt.msc, and click OK. Expand the Sound, video and game controllers list, right-click on the audio driver, and select Update Driver Software. Use the Browse option to navigate to the downloaded package, select the driver, and install it following the on-screen instructions. Next, right-click the Sound icon from System Tray and select Playback devices. Right-click anywhere in the opened tab, select Show disabled devices, click the audio device, and press Set Default. After that, open the Properties window and select Use this device (Enable) as device usage. Do the same with the Recording devices.
If the problem persists, try updating Skype to the latest version. If this doesn't help, I recommend that you contact Skype Support team.