Somebody (or some program) may have inadvertently turned on unwanted audio enhancements in the Windows control panel. Here's how to turn them off:
1. Right-click on the small volume contol in your Windows task bar and select Playback Devices, or go into the Windows control panel, in the Sounds section, and select "Manage Audio Devices."
2. Next, right-click on your playback device and select Properties.
3. On the "Enhancements" tab, put a check mark next to "Disable all sound effects."
4. Click OK to accept this change and see if the problem is gone.