Many computers are shipped with onboard sound cards that use the Realtek High Definition Audio drivers. By default, the recording option is disabled meaning that is not possible to record your computer sound with PlayIt Recorder.
You can enable this recording device with the following steps:
Find the Volume icon on the bottom right, right click and choose recording devices.
You may be presented with something like the following:
Right click the list and choose Show Disabled Devices. You will see Stereo Mix which appears as a disabled device:
Right click this device and choose Enable.
Start PlayIt Recorder and Stereo Mix will be able to record the sounds from your computer. If PlayIt Recorder is already running, make sure you completely Exit the program by right-clicking the Recorder icon on the bottom right and choose Exit.