Hello and welcome to the forum.
Checking here
http://www.dell.com/support/home/us/en/19/product-support/product/inspiron-17r-5720/configuration
I find.......
Product not tested for Windows 10 upgrade
Dell has not confirmed that this product can upgrade to Windows 10. Some drivers, firmware, applications, and connected devices may not work as you would expect.
So I have to ask, did you use the Get Windows 10 (GWX.exe) icon in the bottom right corner near the clock to check for compatibility issues, prior to upgrading to Windows 10?
What does device manager say about your installed "Sound, video and game controllers"?
What is there?
Are there any indications in device manager of "Other" items or device marked with problem indicators?
If it's a driver issue then you can go to dell support and plug in your service tag number and go to drivers and software and you may have to switch to Windows 8.1 in the OS selection to get
IDT High-Definition Audio driver
Other than that, right click the speaker icon down near the clock and choose playback devices and confirm that your "Speakers" are set to default and associate with the correct device.
In my case it says "Speakers" and then SB Audigy right below that.